Excel Based Erlang Calculator for Contact Centres – with Maximum Occupancy

Erlang Calculator v6 Featured Image

Call Centre Helper has released a new version of our popular Excel Erlang Calculator. This version allows you to set a Maximum Occupancy figure.

Version 6.0 of the Call Centre Helper Erlang Calculator

Click here to download the latest version.

The biggest addition is that it now includes the ability to set a Maximum Occupancy figure as well as Shrinkage.

This brings the functionality in line with the online Erlang Calculator.

Maximum Occupancy

One of the big factors that is often overlooked in resource planning is that of Maximum Occupancy.

Occupancy is a measure of how active your contact centre advisors are across and hour. If you try to take this figure over 85% for any long period of time, you will run into difficulties.

The key problems with taking Occupancy above 85% are

  • Advisor Burnout
  • Higher Absence
  • Higher Attrition
  • Longer Average Handling Times

This article provides a good primer on the subject.

Previous Versions

Version 5.5

  • Bug fix affecting calculations below 1 erlang of traffic intensity

Version 5.4

  • Bug fix affecting calculations above 150 agents
  • Amended shrinkage data entry validation from 0.000001 to 0 so can enter 0%

Version 5.3   Removed Erlang C function and replaced with Probcall waits to remove number overload and match online calculator

Version 5.2

  • Bug Fix – Fixed issue with error if period over 546 minutes – (Integer to Long in Excel VBA)
  • Added warning that calculator only works up to 600 agents with advice to use online calculator for large agent sizes
  • Added links to other Call Centre Helper tools

Included in version 5.1

  • Renamed and improved Erlang C Formulas
  • Agents Required – This is the raw figure without Shrinkage.
  • Agents FTE Required – This is more accurate as it includes Shrinkage
  • Service Level Calculation – Works out the service level for a given number of calls and agents
  • Probability that a Call Waits – Works out the probability that a call waits for a given number of calls and agents
  • How to use Tab with details of how the formulas work
  • Help on function arguments by pressing the fx (Insert Function) button
  • Agent Planner – Tab see at a glance the agents needed across the day – with graph
  • Expected Service Level Planner Tab – show expected Service Level across the day – with graph
  • Improved error checking to not allow values outside of expected limits
  • Day Planner and Expected Service Level charts set dynamically – so you can add on extra rows
  • Change of terminology from Call Duration to Average Handling Time
  • Added in missing Help Details when you press the fx button
  • Correction of typos

Shrinkage is a factor used in staffing calculations to take account of holiday, sickness and internal factors. For a full description of Shrinkage, read our article on How to Calculate Contact Centre Shrinkage.

Click here to download the Erlang calculator.

How Does the Erlang Calculator Work?

There are Three Formulas in the Calculator

Please note that the names of these formulas have changed from earlier versions of the Erlang Calculator.

1. The Number of Agents Required

=AgentsRequired (calls, reportingPeriodMinutes, averageHandlingTime, serviceLevelPercent, serviceLevelTime, maxOccupancyPercent, shrinkagePercent)

2. Service Level

= ServiceLevel ( calls, reportingPeriodMinutes, averageHandlingTime, serviceLevelTime, agents)

3. Probability a Call Waits

= ProbCallWaits ( calls, reportingPeriodMinutes, averageHandlingTime, agents)

Worked Example

So for example you have 400 Calls per half hour (30 minutes) with an AHT of 30 seconds and you want to meet a service level of 80% of calls handled in 20 seconds, with a Maximum Occupancy of 85% and 30% Shrinkage.

In this case use the formula

= AgentsRequired ( 400, 30, 257, 80%, 20, 85%, 30%)

Which will give the result of 97 Agents required for this half hour period.

Click here to download the latest version.

The calculator uses macros in Excel so you will need to have these enabled.

The calculator is free of change to download to all of our readers. You just need to add in the unlock code found at the bottom of our newsletter.

If you no longer get our newsletter, it is very easy to subscribe.

To get the Erlang Calculator to work you will need to have macros enabled on your computer.  

In Excel select Tools > Options > Security > Macro Security and then select Medium.

Macro security in Excel

You can then “Enable macro” when you open the spreadsheet.

The calculator is based on Microsoft Excel with a bit of nifty software programming from Joanne Sparkes at Expedio Spreadsheets using a mathematical technique called Erlang Theory*. Jo has coded it from scratch and it seems intuitive to use.  We have upgraded it over the years as a result of user feedback.

Can I Use This Erlang Calculator for a Chat Environment Using 4 Weeks’ Previous NCH Webchat as This Week’s Volume?

An Erlang calculator is not really the best solution for concurrent webchats, but in the absence of anything else it will at least give a rough estimate.

The Number You Divide by Will Need to be Lower Than the Number of Concurrent Chats

The number that you divide by will need to be lower than the number of concurrent chats, as sometimes the agent will be only handling one chat.

So if you had 3 concurrent chats then you would need to divide the number by say 2.5 instead of 3. You can then see how this compares with past patterns.

Need to Calculate the Number of Agents for Webchat and Emails?

We have developed a Multi-Channel call centre calculator which can mix calls emails and webchat. It can also be used as a standalone calculator. It has a really nice visualisation that shows the contacts and the calls over time.

Click here for the Multi-Channel call email and web chat calculator.

What is Erlang?

Erlang Theory  is named after the Danish mathematician Agner Krarup Erlang (1 January 1878 – 3 February 1929) , who invented the fields of traffic engineering and queueing theory.

He published his Erlang C formula in 1917 and it has been widely used ever since.  The Erlang C formula (which is the one that this Erlang Calculator is based on) is the one used for working out the numbers of agents needed for a given call volume. There is another Erlang formula, Erlang B, which works out the number of telephone lines that you need for a given number of agents.

You are free to use the Free Erlang C Calculator in all of your projects, provided that you do not resell or distribute the calculator on the web.

The Erlang C model is used in all major call centre forecasting and workforce optimisation (WFO) and workforce management (WFM) solutions available today.

Terms and Conditions

Use of the Erlang Calculator is subject to our standard terms and conditions.

We also have an online Erlang C Calculator

If you prefer, we also have an online version available. Click for the online Erlang C call centre staffing calculator and here for the spreadsheet version.

In the event that you spot any errors, please leave us some information about them in an email to Call Centre Helper

Author: Jonty Pearce
Reviewed by: Jo Robinson

Published On: 20th Apr 2022 - Last modified: 28th Feb 2024
Read more about - Essential Call Centre Tools, , , , ,

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  • Now that’s neat!

    Dave Appleby 8 Jan at 16:19
  • Jo,

    Erlang seems to work best at 1/4 or 1/2 hr intervals for forecasting accuracy.

    As you get more and more granualr with the data you actually drill down too far and end up in the unfortunate situation of actually trying to look too closely at the Erlang-Poisson distribution.

    For a pure scheduling layout I’m not sure you want to get to scheduling breaks on the 5 or 10 minute level anyway, that would depend on your staff breaking at *exactly* the correct time. With 1/4 hr or even better 1/2 hr schedules you have a little flexability, getting too involved means you are constantly man-managing breaks and fire fighting as the adherence to rota slips.

    Personally I feel it’s over engineering the situation.That said Jonty’s suggestion will work, you do however need to weight the calls toward the front of the 1/2 or 1/4hr to allow for human nature in play (People tend to call just after the hour or half hour, and to a lesser extent after the 1/4’s.



    Dave Appleby 27 Jan at 14:52
  • i need erlang for contact center for forcasting tools.

    Amel 7 Feb at 14:17
  • Amel

    Simply download the Erlang Calculator and you should be able to start making your own forecasting spreadsheets.

    jonty pearce 9 Feb at 18:24
  • I used the excel and it is very useful,but i faced a problem.When i tried to caculate how many agents I need to acheive service level 99.9% and the incoming call rate is only one call and the average call duration is 150 seconds,and the target answer time is 20 seconds, so please feed me back as it is logid if you have two agents and there is only one incoming call,the service level should be 100%.
    So please I will appreciate your help in clarifying that to me.

    Mahmoud Abdel Bar 13 Feb at 17:35
  • Mahmoud.

    One of the causes may be rounding error, the other is that the P.M.F of the POISSON / ERLANG sort are VERY unlikely to give a certainty. The rounding error would be my first choice. The function will say OK I have one call of X duration in Y frame so I need Z staff to achieve the required service level. However, there is ALWAYS some uncertanty.

    As a rule of thumb anyone involved in Statistical / probability is reluctant to EVER say anything is either certain or will never happen, I mean, New Kids on the Block coming back. What’s the probaility of that?

    Below is a breakdown I wrote for elsewhere.

    Erlang being a standard adaptation of the Poisson process is a discrete probability distribution and works on events being independant. It models the P 0.95 for a single call it will indicate two staff are required.

    Generally it’s very good for medium to high volumes over a given time period. Anything more than 5 calls OR 15min talk in a 1/2 hr should be 90%+ accurate.

    The caveat for the whole thing is..

    It’s only as good as the data you’re using.

    The old addage Garbage In = Garbage out still applies.

    Erlang models given P < 1 that a call will come in within a given time period. 1/2hr being best 1/4 works too. Anything else is either too granualar or gross and the P.M.F (probability mass function ) breaks down. It all depends on HOW good you want the svc levels, it will always (in raw unmodified state) give a probability of a second call coming in, therefore even if P> 0.95 for a single call it will indicate two staff are required.

    Hope it helps

    Dave Appleby 13 Feb at 19:03
  • What do you mean by the term “Target Answer Time”, can some one elaborate on this ?

    Sameer Junaid 23 Feb at 11:29
  • Sameer.

    Simply “Target Answer Time” is the time you wish
    calls to be answered within.

    Normally 16 / 20 / 25 / 30 seconds.

    This leads to the common short hand for service levels
    of 80/20 or 85/16 so 80% within 20 seconds or 85% within
    16 seconds etc..

    Hope it helps


    Dave Appleby 23 Feb at 11:33
  • Thanks DaveA for your quick response,

    I think that I got your point of view, for example if an Agent is required to answer a call with-in 3 rings, i.e. 4 seconds per ring, equallling with in 12 seconds. hence, my TAT (target answer time) will be 12 seceonds.

    please confirm if i am right.

    Thanks & Best Regards.

    Sameer Junaid 23 Feb at 11:48
  • Exactly.

    The industry standards tend to be between 10 and
    20 seconds.



    Dave Appleby 23 Feb at 14:25
  • Thanks a lot DaveA

    Thanks & Best Regards.

    Sameer Junaid 24 Feb at 05:53
  • Is your calculator capable to calculate delay data i.e. number of callers waiting longer than x seconds?

    Cornelius Timothy 25 Feb at 21:27
  • Cornelius,

    One of the problems you would have is that
    Erlang, by it’s nature, is geared toward answering

    It actually works assuming the maximum time you are
    prepared for a caller to wait.

    In theory you should be able to say that if you staff
    correctly and pick a sensible target answer time callers
    shouldn’t have to wait, however, we do know that this
    isn’t the ideal world.

    Having recently been asked something similar and to calculate staffing for various abandon rates and it’s
    not as easy as it seems.

    Not exacly what you’re after I know..



    Dave Appleby 26 Feb at 09:09
  • Can you elaborate on what calculation the user defined function “agent no” uses?

    Lou 1 Mar at 15:16
  • Lou,

    The UDF is the actual staff required calculation based
    on call length and service level required.

    It actually loops incrementing the agent
    number until the target is hit. The actual function
    is driven by the Erlang UDF.

    If you have a look at the Vba in the background
    it’s the last function in the Module.

    Hope it helps


    Dave Appleby 2 Mar at 13:00
  • Hi Dave,

    I have macro security disabled entirely and still can’t get the whole page to work. The daily half hour breakdown does not calculate.

    I had built a very similar tool at my previous place of work, though it required the Erlang for Excel add-in. Is that required here?

    If so, are you able to email it to me?

    Thanks for you help.

    Scott Allen 27 Mar at 17:32
  • Scott here again – not sure what did it, but after clicking away and back a few times, it worked.

    Looks great. Thanks again.

    Scott Allen 27 Mar at 18:08
  • Scott,

    Can you let us know which version of Excel you’re using?

    It sounds like there’s an issue with the automatic calculations.

    I’ve not seen this with the calculator, however, I have
    seen it with other UDF’s in, paradoxically, Excel97 and 2007. ALthough not 2002 et al…



    Dave Appleby 27 Mar at 19:48
  • In the first box “Calculate the number of agents required to reach an agreed service level:”, do you put in the average half hourly volume per day or the max half hourly volume? Good calculator though.

    Claire Shirley 1 Apr at 13:06
  • Also, does that include shrinkage?

    Claire Shirley 1 Apr at 13:08
  • Do you have the staff utilasation excel formula

    Peter 5 May at 17:11
  • OK,

    Been away for a while….

    Claire: It will give you agents required on a raw basis,
    ie: this is what you need staffed at any given interval. Shrinkage is a bit too dependant on local variables to be factored into such as this

    Peter: Utilistaion% is :-

    (Talk + After Call Work + Shrinkage factor)/Login *100
    I would suggest a productivity figure of 65-70 % to aim for.

    Regards all…


    Dave Appleby 6 May at 07:49
  • How would I use this tool to get an annual FTE required?

    Rhonda Haddock 22 May at 17:41
  • Agents are supposed to login for 9 hrs but due to any particular reason (eg CAB delay), Agent is logging only for 8 hrs. how can i calculate it on excel file, please suggest.

    Siddharth 2 Jun at 23:56
  • If I have my staffing and workload is it possible to use erlang to calculate what 80% of customers will be answered in? (ie not using a service level target)

    Kerrie 22 Jun at 01:28
  • I would like to paste the calculator into an existing spreadsheet that has my call volume data. When I paste it though the “Numner of Agents Required” box shows #/Name (and the formula itself is gone.
    I am able to downlaod and sucessfully use the calculator in its downloaded Excel file, jsut have the problem after the paste (I checked and Macros are enabled on the paste to file)

    Thank you
    Gregg Solomon

    gregg solomon 23 Jun at 21:02
  • Gregg

    Jo has had a look through the code and has come up with the following pointers.

    The reason they have disappeared is because all the formulae are based on user defined functions (rather than built in Excel functions). These do not show in the visible macros but can only be seen by going to Tools, Macros, Visual Basic Editor and then looking in Module 1. They won’t be transferred in a standard copy and paste of the spreadsheet.

    It should be possible to manually do a copy and paste of the text in module 1 to module 1 of the visual basic editor in his new spreadsheet and then the function “agentno” will work. Otherwise, depending on the volume of data in his other spreadsheet, it may be simpler to copy the data in to new sheets in the Erlang spreadsheet.


    jonty pearce 25 Jun at 09:24
  • How do you enable Macros in excel version of 2007? I don’t seem to fidn it.

    Samuel 7 Jul at 05:30
  • I have made all the changes and settign adjustments and then tried to enter the required inforamtion, nothing changes and i don’t get what i want. what else am i supposed to do?

    Samuel 7 Jul at 05:50
  • Samuel,

    A little more info would be useful…

    HAve you managed to enable Macros?

    Is it giving an error message?

    Have you opened the whole file or just copied
    it into another worksheet?

    let us know..

    Dave Appleby 7 Jul at 22:18
  • can you please explain me what formula did you use to get the required agents field? and the predicted service level? this would help me understand better how you was this done.. Thank you in advance,

    JC 6 Aug at 22:46
  • I prefer using computer simulators as Erlang assumes certain important activities. Anyone knows where to download a computer simulator software in excel?

    Luther 7 Aug at 07:45
  • hi,

    just wondering if there is a similar tool for non-voice contact centers. specifically a calculator to determine “threshold” manpower limit for a fixed volume of documents to be processed. tx!

    Maynard 19 Aug at 13:35
  • Something like Nuqleus will probably be what you’re
    after, although that’s really a back office MI app.



    Dave Appleby 19 Aug at 14:03
  • tx for the tip DaveA.

    however,are there simpler excelbased spreadsheets. will use it more for basic stuff, specifically determining “threshold” levels for manpower before SLA production volume suffers. tx!

    Maynard 20 Aug at 02:32
  • Hi, can some one please tell me that the “Average Call Time” refers to “Average TALK time” or “Average HANDLE time” ?

    Nauman Cheema 20 Aug at 07:34
  • Nauman,
    It refers to the total time required to handle the call
    including any after call work required.



    Dave Appleby 20 Aug at 13:29
  • Shrinkage isn’t factored into the model, right?

    Nathan Fish 25 Aug at 15:27
  • Nathan,

    See my comments above on the 6th May
    regarding shrinkage as a local variable.


    Dave Appleby 25 Aug at 17:28
  • Thanks, this an excellent tool. For those wishing to convert to 15 minute intervals, you can do this by changing the denominator in column G from 1800 to 900, then inputting data in 15-minute increments.

    Nathan Fish 25 Aug at 19:32
  • Hi ,

    I have downloaded your calculator which is working properly. Thanks a lot .

    I have a one question.

    Why did you divide =I14 / 1800
    why we are using 1800.
    Could you explain . please

    Kivanc Solak 22 Oct at 18:16
  • Kivanc,

    Simply put, it’s the number of seconds in half an hour.



    Dave Appleby 22 Oct at 18:34
  • Can anyone give an explanation of what the ‘Traffic Intensity (Erlang)’ means. If it’s increased, the performance level falls. Thanks!

    David Blackman 12 Jan at 11:40
  • David,

    Essentially and ‘Erlang’ is a descriptive unit of measurement in the same way as a Newton or Watt. It describes the loading on a telephony system.

    Effectivly it is Time available on a system so…

    Time interval / (Calls*average duration)

    3 calls of 20 minutes in an hour would be one Erlang

    3 calls of 15 minutes in an hour would be 0.75 of an Erlang

    15 calls of 23 minutes in 2 hours would be 2.85 Erlangs

    Anything greater than 1 means there is more traffic than one line can take.

    To answer your main question, if demang goes up service goes down fr the same number of staff. You do need to remember that with Traffic intensity, it is an average over a period of time and not indicitive of any queuing behaviour in teh system.

    Hope it helps.


    Dave Appleby 12 Jan at 11:57
  • Hi,
    Please provide me with the exact formula for calculating Shrinkage % and Present % .
    Thnaks !
    Ravi Verma

    Ravi Verma 26 Jan at 13:46
  • Hi Dave,

    Is Erlang suitable to estimate agents for email queues, where the SLA may be 2 hours? So in the calculator if the SLA is set to 95% in 6480 seconds (7200 sec – 720 to complete the effort to respond to the email) would it produce a viable result?

    Would you recommend another approach?

    Mit 3 Feb at 16:38
  • When using this model am I correct in assuming this is core number of bums on seats not allowing for shrinkage thanks

    Marie 4 Feb at 11:59
  • Marie,

    Spot on 🙂


    You can only factor shrinkage in IF you know what it
    is for your particular centre, there is no standard
    formula I’m afraid.


    It will work IF you fator the queue / Svc time the way
    you suggested, although a more ‘analogue’ system may
    work as well in this situation.

    Regards to all..

    Dave Appleby 4 Feb at 14:00
  • Does the incoming call rate include or exclude abandoned calls?

    Thanks for your excellent tool!

    Ian Logan 18 Feb at 14:57
  • Hi,

    This is a great calculator.
    However, can some1 please inform me the formula to calculate FTE with AHT, Service Level, and Shrinkage.

    For example:
    Our company operates Mon-Fri, 8am-8pm;
    Average calls per day is 2000;
    Service Level is 85%
    Shrinkage set to 35%
    AHT is 310.
    How can I calculate the FTE now??
    Please help.



    Weera 3 Mar at 11:48
  • Hi,

    how do I calculate ‘Calls per hour’,required for ‘SLA’ calculation,if I have monthly call volumes.I am trying to figure out what will be my Service level for the month.

    Sandy 25 Mar at 08:23
  • I just downloaded this calculator it seems very interesting and most of my difficulties related to center staffing seems to come to an end today….. Lets see how effectively it will work….

    Nitesh Ajmera 19 Apr at 18:07
  • Hi,

    Can you define me WFM service level formula please.


    Abdul 28 Apr at 13:05
  • Abdul,

    Basically the simplest formula is:-

    Of x calls answered what percentage were
    answered within the target time of y seconds.



    Dave Appleby 28 Apr at 13:28
  • Hello

    Is Erlang suitable to estimate agents for email queues, where the SLA may be 2 hours? So in the calculator if the SLA is set to 100% in 7200 seconds, would it produce a viable result?

    Would you recommend another approach?

    Regards – Sayan

    Sayan 27 May at 11:07
  • Do you know what happens to Erlang if you have a hybrid of two different call types with different qualities of service? For instance, we have all of the regular help desk calls that have to be serviced at 90% answered in <45 seconds, but a subset of same call center workforce also handles another split/skill with service level requirements at 95% at < 30 seconds. I don’t think that that would be an additive thing. How would you figure the right staffing? In other words, can erlang handle 2 service levels for 1 call center and calculate total required staffing? Thanks.

    Bill 11 Jun at 12:45
  • Hi!

    I have very low number of incoming calls per hour, around 3-4, and between 200-500 seconds of handling time per call.

    – Can I use the Erlang calculator for these values, or should I rather calculate on number of agents needed ideally without taking service levels into account?

    Camilo from Sweden

    Camilo 16 Jun at 16:49
  • How do I find the VB code for this tool?

    Teddy 27 Jul at 13:14
  • Hi Dave,

    This tool is pretty good. Assuming we have multiple language skill sets with varied call volumes, do we have to assess the headcount required for each language skill set as different entity. I have to support 9 languages


    Deepak Vittala 29 Jul at 10:52
  • Hi Deepak
    Your answer will depend on the degree to which your staff are multilingual. If you have 9 languages and everyone is multilingual, then you can use erlang on one worktype/skilltype.
    If you have a more complex situation of some people being single and some multi-skilled, then simulation is the best route, but more complicated.
    Failing that you can estimate how may skill groups you have, say 3 if in your case the average number of languages spoken was three. And then break your call demand into three, and run each through an erlang model. Not perfect, but can be quicker and simpler than full on simulation.
    Hope that makes sense. Let me know if it does not.
    Best regards, Steve

    Steve Connolly 29 Jul at 13:34
  • Thank you Steve for the suggestion. I am awaiting information on the skill groups so that I can estimate the headcount. Do i have to add any buffer head count or the Erlong model would take care of it. Also the support is not 24×7 support. The support window is just for 9 hours window.

    Deepak Vittala 29 Jul at 15:08
  • Good calculator.

    If you wish to use the model by entering higher level numbers – such as calls received per hour or calls received per day, you can change cells D14 and E14 by switching 1800 with the number of seconds in your timeframe (per hour would be 3600).


    John 25 Aug at 13:12
  • I was looking for similar tool which I can use to plan my resources for back office process. Can anybody help me with this tool? Where I can find this tool? I’ve prepared the planner which includes the incoming volume and RPH (Rate Per Hour) but I was looking for some standard format.

    Pravin 14 Sep at 15:26
  • Hello.
    Does anybody know to estimate the call volume demand when you implement a new system – ERP (SAP , Oracle, JDE) for example 3 calls a month per user ?

    Miguel 22 Sep at 22:16
  • Hi
    How can i please change the VBA function for agentno so that it returns decimals?


    Alice 28 Sep at 13:08
  • Is there any way to get this to work based on call answer rate rather than service level?


    Mark 26 Nov at 18:06
  • Hi,

    Read above you cannot use calcualtor to forecast split service levels but can you forecast and include a max call wait time as an output? Basically I need to calculate FTE on GoS 80/20 on 400 calls but I also need to factor in a new SLA of 100/60 on the same calls which is where we current fail to focus attention by only targeting calls still in our SLA. Any ideas?


    Chris 8 Dec at 14:35
  • This is heaven-sent! I’ve been searching for a free Erlang C calculator and I only found out about this just now. I’m not yet done using it but will give my feedback/input as soon as I get my results.
    I’ll post this to my blog as well to help my co-WFAs.

    Avee Balaag 5 Feb at 09:11
  • Hello, I’m ok using the calculator, I just noticed one formula. The one above the Incoming call rate at the left hand box. it says D14/1800. I’m just wondering where you got the 1800? Is that variable or fixed? Thanks for your help!

    Avee Balaag 5 Feb at 10:48
  • 1800 is the number of seconds in half an hour, this is the interval the calculator works to.


    Dave 5 Feb at 13:12
  • This calculator is fantastic for identifing interval requirements across the day. I’ve read many threads in quite a few forums and found several formulas for working out required FTE and I’m almost there but I’d really like the excel formula for working out required FTE for day/week/mth call volumes.
    Using the following data day/wk/mth e.g.
    Calls day/wk/mth – 1198/6049/23058
    AHT – 440 secs
    Occ – 82%
    SLA – 85/60
    Shrinkage – 22%
    Hrs worked/day – 7.5

    Looking forward to any assistance.

    Nigel 23 Feb at 01:45
  • hy
    i must calculate input time of the random time .
    your help me
    i am iranian i dont english
    thank you

    fatemeh 6 Mar at 10:26
  • Hi Dave,

    I am a WFM Analyst here in Dubai and I’ve been a fan of Joanne’s VB codes for the past year.
    I’ve been wondering though what is the relationship between Intensity,ASA,AHT,and SLA on the below formula..


    I love spreadsheets but i never had an experience in writing and understanding VB codes… once i see “Dim agents As Long” etc i loose my interest and close the file.

    Is there anyway you can explain
    =AgentNo(Intensity,ASA,AHT,SLA)in a layman’s term like using M.D.A.S.. I can understand Algebra and Trigonometry more than VB codes.. =)



    Drebb, Dubai 15 Mar at 20:55
  • Hi
    The erlang calculator looks great and is working -however, I am looking for a calculator to help forecast weather related calls. These calls are not predictable – would this calculator be of any benefit to me. I am having great diffilculty in producing a forecasting model that is within 8/10% accurate.

    Many thanks


    Lynne Marie 29 Mar at 18:46
  • LM,

    It really depends on the kind of lead time you have, and, your trigger events are they sudden or do you have any lead time at all?.

    A slightly more sophisticted model may be of more use, as, you’ll need to factor the sudden uplifts.

    That said, if you are weather dependent you’re constrained by triggers, so, can probably build a lookup of…
    Event A = xxx calls with a distibution of yyy minutes / hours.

    Event B = ppp calls in qqq minutes / hours.

    You can then factor that into the model and look at the staffing requirements for the sopt intervals. That’s something this calculator can do.



    Dave Appleby 30 Mar at 07:23
  • The below Erlang function doesnt match with the Actual ASA output with the given parameter
    For example if i want to validate the actual ASA delivered with the given Agents, calls and AHT i get different output

    YR 7 Apr at 21:03
  • YR,

    Can you clarify a bit more, not sure what
    you’re driving at.

    When you say you go to validate, how do you validate?


    Dave Appleby 8 Apr at 12:03
  • Hi Dave

    Many thanks for the advice on 30th March – sorry I have not replied earlier. I will certainly look at this and let you know how we get on.


    Lynne 10 Apr at 23:57
  • Hi Dave,

    If I wanted to use the total volumne of calls instead, would I just increase D14/1800 to the number of seconds for the week?

    Andrew Z. 3 May at 16:57
  • Dave,

    I know this has been asked a couple of times but havn’t been able to see the answer… can this be used on for a daily, weekly or monthly call volume requirement?



    DSL 11 May at 20:34
  • Sorry for the delay,
    I’ll try and answer both questions at once…

    Theoretically you can upscale the interval data, but, this destroys the granularity and really attacks one of the strengths of the Erlang model.

    Uplifting to a one hour interval will work fine, however, when looking at at a week looking at 216000 seconds based on a 12/5 as your forecast plan will only ever give you a ballpark figure asthe daily /interval trends are washed out.

    This error gets larger as you increase the interval, with a monthly you’ll lose the weekly trend throughout the month.

    What that IS good for that tough is looking at longer term planning and advance scheduling of major training etc… What you can do is look at the variance, match it back and then see how big a gap is developing. You can then apply that to say the monthly plan and look at available days etc…

    Hope that helps, drop a line back if you’d like anything more.



    Dave Appleby 12 May at 08:10
  • Thanks DaveA that certainly helps and is logical. I have a quick question about the agent planner – this is based staff (without shrinkage) as bums in seats… Once you add our 40% shrinkage in to get our rostered staff, what do you propose as been the best way to calculate total staffing needs over a 15/5 (40 hr week with 5*8 and 4*10 schedules)?

    DSL 12 May at 16:26
  • what type of occupance is calculated in or is it simply the number of poeple you need at any given interval to be available to take calls? I want to run it with 82% occupance and want to make sure I’m not under estimating my staff.


    Alexander 24 May at 17:51
  • Hi Dave,

    I just wanted to say this is a great spreadsheet.

    You are also very patient in answering all the questions above – very detailed! (even for repetitive ones)

    Greatly appreciate your hard work!


    Joanne 26 May at 16:25
  • Loving the calculator and I would love to able to use the formula in a spreadsheet we have that tracks call volumes and plots this against a planned rota to highlight any potential gaps in our resource levels, but I cannot get excel to recognise the ‘agentno’ in the formula. Can you let me know which add-in I need to be able to get this to work.

    thanks in advance for your help

    Sarah 27 May at 13:36
  • The Calculator is accurate, however i need to know the exact calculation of formula “agentno” & “Erlangc” used in the excel sheet.

    Nilesh T
    +91 98 234 234 15

    Nilesh 19 Jun at 04:46
  • I am trying to find a fomula that will help to calculate workload based AHT or OPHA (Outlets Processed per Hour (Average). It’s for an Outbound environment. We know how many calls we have for the day,how many agents are working each hour,AHT and Average Dials. Just wondering if anyone has experience with Outbound Daily Forecasting

    Jonathan Freel 5 Jul at 21:42
  • Hi – has there been an update so we can calculate the agent needs per 15 minutes? I am currently trying to fix the call volme planning / forecasting for my contact centre and I am slightly struggling. I need accurate figures per 15 minutes. My idea is the following:

    Pull Volume figures (Calls offered) and Handling Time (AHT) over the last 3 years. This data will help my forecasting (simple calculation of trend, should be fairly accurate as far as I could tell). I will hopefully get figures for Calls Offered / 15 minuntes including AHT separately. Based on this info combined with Service Level I should then hopefully get correct staffing requirements which will help me plan the days and weeks ahead.

    Any tips or input is very welcome. Thank you for having put this Erlang C Calculator online, it is a great help!

    Regards, Andy

    Andy McMullen 18 Jul at 12:03
  • Hey – I’m trying to build a staffing model and I’m struggling – does anyone have any good examples of how to go about doing this. I understand that the calculater will ultimately tell me how many people I need depending on the variable inputs that I use; however, I want to take a step further and use the calculator to show me what my expected rates should be etc. etc. – just want to build something that has some functionality. I’m also looking to show this in 15min intervels. Any help would be greatly appreciated. Thanks!

    erlangstruggle 23 Aug at 18:57
  • Hi Need a simple calculation to know the number of agents required to answer 100 calls with an AHT of 12mins and shift of 9 hrs.

    Kapil Rajpal Singh 7 Oct at 08:55
  • OK,

    The calculator will do that if you know the distribution of calls.



    DaveA 7 Oct at 10:40
  • Dave,
    You are a bit of a hero for both the calculator and your time to answer everyone’s questions.

    Could you answer one more?

    I am struggling with the Erlang cell.
    I read your reply above, but can’t get it right in my head.

    Itis set at ’20’ in the calculator…. Can you explain in layman’s terms what I should put in that cell please!??

    Can’t thank you enough.
    If you take donations, I’ll happily forward one.

    Allan 18 Oct at 22:18
  • Hi
    When I created a new module in my worksheet, and agentno function calculated there 18 agents for 19 calls in 15 min, AHT=121, target answer time=10, SL=77. But in native file same calculation shows 7 agents. How can it happen?
    Best Regards,

    Cumshud 22 Oct at 20:58
  • OK….

    Couple of answers(ish)

    #1 Cumshaud.
    I can’t really support anything outside the main download, I’d guess you’ve a reference wrong somewhere though. If it’s working fine in the native code can’t you leave it there?

    #2 Allan.
    Which cell are you referring to? The cell set to 20 as default is the target answer time in seconds, or, am I missing something?



    Dave Appleby 24 Oct at 11:35
  • Thanks for this calculator! Is there a way to vary an agent’s “productivity?” (i.e., the amount of his/her an agent would really be able to take calls. This calculation would include biobreaks, time to get a drink of water, etc. Also, to add “Wrap-up” time, would I ismply increment the call length to include, for exampole, 15 seconds between calls?

    Pascalpal 25 Oct at 17:09
  • Ok,

    Wrap should really be included in the AHT for the call not just talk, so, that’s
    not an additional factor it’s part of the requirement.

    From a calculator like this you need to factor your
    ‘shrinkage’, IOT (Indirect operational time),
    productivity or what ever you’re calling it as an
    additional factor above and beyond what the
    calculator says.

    This just gives you the ‘raw’ number required at any
    given interval.



    applebyd 25 Oct at 18:50
  • Thank you!

    Pascalpal 25 Oct at 18:59
  • Hi dave,

    You still have not answered how you obtained the number of agents…

    Can you explain it in simple math.


    omaiaa0p 2 Nov at 08:40
  • Sorry,

    Not sure what you mean.

    I can’t find a previous query from you.

    If you mean in general how do we find the number of agents, it’s based on the Erlang C distribution calculations which in turn are a derivation of the Poisson distribution.

    The Wikipedia page is a good place to start.


    As always with Wikipedia, don’t trust it as the only source! Look at the references and go from there.

    The basic theory in obtaining the number of agents is:

    1) Look at the historical call arrival and handle time.

    2) Use the probability model to calculate Agents required based on the base data.

    Unfortunately it’s not a simple calculation, the upside is there are plenty of tools, like this one, available to help.



    DaveA 2 Nov at 12:34
  • would u pls tell me if landed calls 500000 and AHT(avg handling time)500 wht should be the manpower forcasting with 40% shrinkage? productive hour:8 hr

    nazia 3 Nov at 05:39
  • Nazia

    That’s what the calculator is for:-)


    DaveA 3 Nov at 08:20
  • Hey!

    Can I calculate staff breaks with this program anyhow? If not, can anyone tell me how can I calculate staff breaks in excel?


    oiro 12 Nov at 19:30
  • Hi, can this be used to calculate agents required to do email servicing? Can it also show productivity %?


    Aida K 29 Nov at 11:31
  • Just a quick question if I may,

    when inputting available agents into the erlang form, doe’s it allow for breaks or would I need to adjust accordingly, silly question I know.

    Your help in this matter is much appreciated.



    Jez 16 Dec at 16:55
  • Hello,

    Thank you for this awesome calculator. My CSR’s avergage about 19min / call, I have SLA set at 120 seconds, on a busy monday, we receive about 800 – 900 calls, is 120 seconds too low or should I increase to 180 seconds? My dept ends up abandoning about 20% of incoming calls.

    Fidel 1 Mar at 21:14
  • Quick question. The calculator provides required heads per interval. How do I convert that into required hours per interval?


    Kathy 9 May at 18:41
  • Kathy,

    A head will equal 1800 seconds, so…

    Multiply heads by 1800 then divide by 3600…


    dave appleby 9 May at 19:23
  • @Fidel, Have you work with the inefficiency of your team?

    My dpt receive at least 1200 calls in a single monday, with a SLA 90/30 and 20 min AHT and my SLA is up to 95% Abd rate is between 0.5% and 1%

    Sometimes it’s not only to increase the number it’s to handle an efficient team.

    Now also is important to know how many agents do you have and how many shrinkage you have abs level, that’s a huge analize to do!

    @Dave! Thanks you so Much for create this tool, it’s really usefull , I was working with the oldie erlang97 but your tool it’s more accurate!



    JK 18 May at 01:21
  • Great tool. I see the question asked many times, but no answer…does this factor any intervals between calls, or is asumption that calls follow in immediate succession?? thanks.

    Big Dave Spin 7 Jun at 19:18
  • Hi there,
    Does this tool take into account shrinkage? It seems like it is determining agent requirements but I cant see where shrinkage is factored in??

    Cheers, Tim

    Tim Munns 8 Jun at 05:41
  • OK:

    Couple of answers..

    Big Dave: No, Erlang assumes calls arrive and queue one after the other, so, effectivly a 0 interval.

    Tim: It will give you agents required on a raw basis, ie: this is what you need staffed at any given interval. Shrinkage is a bit too dependant on local variables to be factored into such as this

    General Reply regarding Open Office.


    I’m still looking at this, however, a change in job, and country, has slowed it down.

    It looks like a change in the way 3.2 handles UDF’s but the coding is a subtly different flavour from the MS VBA, I’m working on it..

    Regards to all.


    Dave Appleby 8 Jun at 07:25
  • Hi Dave A,

    I am trying to calculate agent occupancy:

    I have 25 agents spread over 5 shift schedules.
    SL:80% answered within 30secs
    Wrap:30 secs

    what other data that i need to come up so that i can properly calculate occupancy? hope you can help.

    kivz 9 Jun at 22:54
  • Hi Dave,
    I have run the calculator for a week calculating FTE requirements for every intra-day period and the results are far too low. My FTE requirements for the week amounted to 19 FTE (80%/30 seconds) as opposed to 27 FTE using an occupancy based formula with an occupancy target of 80%. I guess an 80/30 SL would not equate with 80% Occ???? I then did a sanity check to see what actual FTE was handling the workload and it was approx 27. Any ideas on how to amend the formula.

    Cheers, Tim

    Tim 15 Jun at 04:27
  • I have two sets of calls. 1. AHT is 230 seconds and 1. AHT is 425 seconds. I have two sets of associates answering different jobs. Example 1 2000 calls with AHT of 230 and 2. 14000 calls wit AHT of 425. 10 agents taking the lower AHT calls and 300 agents taking the 425 aht calls and remaining 230 aht calls that are not getting handled by the other agents. When I calculate by half hour the combination of calls, AHT, and Agents Erlang can not give me an accurate service leve. It does however give me the accurate staff shortage or overage but I may show I am short 15 associates and get an 85 service level when you know it is going to be more like a 55. do you know how I can calculate both?

    Leslie 17 Jun at 16:10
  • Hello, This is a great tool but I’ve made some cosmetic changes that allow me to paste an excel export from my scheduling software onto a 2nd sheet and populates the orginal sheet, and sometimes when I paste it freezes and sometimes it doesn’t. Any idea why this could be happening?

    Bill 27 Jun at 20:09
  • Which will be the conversion factor from 1 Erl/Mbts with a vocoder EVRC, 9.6 Kbts (CDMA system) and a vocoder G729 16kbts (Wimax systems)

    Rey 19 Jul at 02:09
  • OK,

    Reply set….

    1) Tim and Kivz..

    This literally gives your fte demand, assuming the standard Erlang no-blocking functionality.. Any occupancy figures need working out separately. Within the calculation it assumes an occupancy of 1 (ie:100%) for a pre-shrinkage calculation.

    Within the code is a variable called ‘utilisation’
    you can experiment with this, but, I’m not an advocate of mucking around with that bit, and, am probably not going to be able to help with any specific issues incurred by changing that.

    Alternatively have a look here


    Is well worth a read.


    This is a single queue model, trying to work with two will give a good approximation of FTE requirements, but always assumes an even distribution of both types of call. This is probably leading to the SL disagree..


    Excel support time 🙂

    Try pasting values then formats as opposed to
    paste all….

    4) Rey.

    This is a voice traffic calculator, sorry but you’ll need to speak to your vendor regarding VOIP
    bandwidth issues.

    Hope that helps



    Dave Appleby 19 Jul at 06:56
  • Hi DaveA,

    I have been using your calculator for some time now and have managed to manipulate it into a bigger picture spreadsheet that can directly map a live rota (factoring in individual agents availability on a 15 minute basis) into your calculator to be able to make real time tweeks to staffing levels on a daily or weekly basis on the rota that will instantly show the impact on service and staffing needs on a daily and weekly basis.

    It has worked well to date for managing a small to medium sized team (I have created 2 such documents, one for up to 15 staff and 1 for up to 30 staff). The problem I have is that I think there is more capability in what I have designed for larger services, but I do not have the VBA experience or know how to make it work without getting into excessively large spreadsheets!!

    Just wondered if you wanted to see what I had managed to do and see if it was of interest for yourself to make better (?) as others may also benefit from a combined rota/erlang tool as I know we certainly have.

    Please let me know if you fancy the challenge as I’d love to have someone else’s input on what I have managed to do.


    Matt Hedges (@CustServiceMatt) 23 Aug at 12:28
  • Hi Dave,

    I have the calculator open in Excel 2007 and macros enabled. However, no matter what I enter for variables – call rate, duration and service levels the number of agents remains calculated as 1 ! What is causing this?

    Also the model currently shows hours of operation from 7:00 to 15:00. How do I change it to reflect our hours 7:30 to 17:00?

    DonH 23 Aug at 20:09
  • Hi Dave,

    Disregard second question. I can add and delete hours of operation 🙂 Top part of calculator is still not working.

    DonH 23 Aug at 20:31
  • Hi DaveA

    Seem to have resolved 2nd problem – Traffic Intensity (Erlang) formula was missing from uppler left hand calculation box

    DonH 23 Aug at 22:32
  • which grade of secvice is standard for core and wh ich one is for acees?

    Hussein 26 Sep at 05:52
  • Hussein,

    GOS is defined by the user.

    Can you clarify the question please?


    Dave Appleby 26 Sep at 06:45
  • Hi Dave;
    I want to calculate the Erlang capacity based on available circuit in core pare, , I have to consider blocking probability rate, so I need to calculate the Erlang capacity with grade of service from Erlang B table.
    Can you please let me know which GoS is standard?
    0.01% , 0.1% , 1%
    0.02% , 0,2% , 2%

    Also for radio access part as well.


    Hussein 26 Sep at 11:03
  • how can we connect that sheet with auto dialier sofware

    mohsin.raza 27 Nov at 21:24
  • I need to forecast service level by interval and was wondering if erlang could do this ( I know my scheduled staff by interval and wanted to say at 915 we are predicting a service level of 5%)

    Andrew 4 Dec at 11:08
  • Hav a quick look at Col:D rows 38-88.

    There if you put in AHT AND and target Svc level you can then add number of agents in and it will give you teh predicted svc level.

    Should be what you’re after.



    Dave Appleby 4 Dec at 11:51
  • Dave, have you any solution in order to run this calculator in Open Office?


    Walter 18 Jan at 23:03
  • Nice job, this is really helpful!

    Chris B 22 Feb at 18:01
  • Walter,

    sorry. still working.

    which version are you using?


    daveA 23 Feb at 01:45
  • A one hour call from one user equals one erlang?? but how much is the total traffic in MSCS considering the uplink and downlink?? 1 erlang or 2 erlang?

    subham 28 Feb at 11:38
  • OK,

    can you define MSCS please?

    the only references I can find are network
    related, not voice traffic.



    daveA 28 Feb at 14:47
  • Looking for some advice…

    I don’t see where shrinkage is taken into account within the required FTE calc. What is the best way to account for that after obtaining the base line FTE from this calculator?

    Trevor 28 Jun at 18:38
  • Hi Dave,
    Your model rocks! I just read through this page and wow, a lot of good information. I have used your spreadsheet as a base for many models. I start with your spreadsheet, as I could not get the code over(from reading I now understand why). I used it to create a staffing plan, integrated shrinkage formulas, etc.. My thought is that it would be great to have a place on your website where people could offer their creations that are based on your model. I have not seen anything else out there like what you have, and by letting others contribute to a assorted “Collection” of solutions based on it, I think would just be awesome.

    Tim Peyton 12 Aug at 19:13
  • Hi Tim

    That sounds like a great idea.

    Would you be able to mail it in to us and we will add it on.


    jonty pearce 13 Aug at 15:09
  • the agent planner table… i input 6 agent… the incoming call rate column…what should i put? is the incoming call rate consistent?

    Alli 15 Aug at 02:18
  • And finally – Dave – what are this week’s lottery numbers? You manage to answer every other query, so I am sure you can manage this one LOL

    Pennyd 15 Aug at 12:38
  • Alli

    You put in your actual incoming call rate.



    DaveA 16 Aug at 08:18
  • Hi. This is a great little calculator. Could you tell me if it would work on hourly data if I changed the 1800 to 3600? Thanks Mike

    Mike Lewis 30 Sep at 09:53
  • yes,
    Just remember to change the
    times as well 🙂


    DaveA 30 Sep at 09:58
  • When Fractional agent is used to calculate interval level req let’s say @ 80% 20sec target then if the same rq is used to calc SLA..it does not give 80%SL as result? In other words Reverse calculation doesnt hold true.Does any one know why?

    Ritesh Khandelwal 9 Oct at 06:53
  • Also if fractional call cacacity is used to calcuate the call capacity of the agents dervied through fractional agetns it does not revert the same no of calls that were used in fractional agents..Do we know any thing about it?

    Ritesh Khandelwal 9 Oct at 06:56
  • Erlang C and it’s like are Trapdoor / One way algorithms.


    You cannot always reverse the probability equation.



    DaveA 9 Oct at 08:21
  • what is the formula to know how many agents we require in each interval

    in 1 hour we receive 100 calls
    Our call handelling time is set to 10 mins******* how many agents we need at that period to answer all the calls

    Anonymous 13 Nov at 07:48
  • Err…..

    Not sure what you mean.
    The erlang calculator will work
    out your staffing for you.

    The formula is basically:

    Where row d contains the agent figure, G the trafic intensity in Erlangs and H the Productivity / Utilisation required.

    This is just a demo of the Poisson distribution function.


    DaveA 13 Nov at 08:56
  • Ravi,

    Without knowledge of how your calculator functions
    we are going to be unable to ascertain why there is
    a variance.

    I do not know why they vary but am inclined to go with the Excel
    Erlang calculator.


    applebyd 18 Nov at 10:03
  • Hi @Applebyd,

    Currently Im using the Aspect WFM tool in my PC, Im comparing the excel output to the tool Staff Group’s Actual Required without Staff Adjustments and Unproductive, Always there is a difference in the output, some values are exactly matching, but some are having variance, Please Advice on this, any idea about Aspect WFM tool calculation.

    Please help.

    Ravi 18 Nov at 11:24
  • Any one, have tried the above calculation ?, Please Advice


    Ravi 19 Nov at 09:37
  • Ravi,
    The problem is that the ‘raw Excel calculation
    will be different from the propriety one that Aspect uses.

    It is very unlikely that

    a) They will ever match exactly and

    b) Anyone here will know the inner algorithm coding of the Aspect WFM, that’s
    information that companies closely guard as it’s the key driver for their product.

    I think you’re going to have to settle for the fact there are
    differences between Aspect and Excel.



    DaveA 19 Nov at 09:42
  • Can any one help me how to get number of agents , by using erlang Abandon formula.

    Please Help,


    Ravi 19 Nov at 11:14
  • OK,

    Now that’s a different question.

    Erlang doesn’t actually work with Abandoned calls it
    assumes a call will queue indefinitely.

    What you are after is Merlang:

    Most WFM use the approach covered in the simulation
    paragraph therefore, whilst their
    output will be close to Erlang / any formula based system
    their output will never match exactly.



    DaveA 19 Nov at 12:57
  • Hi!

    I have downloaded the Erland C calculator and I wish to use it in Excel 2013.

    When I open your file, it works.
    The instance I click “Enable Macros” it does not work.

    I get #NAME error in any field calculating with the custom formulas.

    I tried saving the file in a newer file type (.xlsm) to no avail.
    I have lowered macro security but I just can’t get arround that #NAME error.

    I hope you are able to help me.
    Is there anything I should be aware of, when using Excel 2013 instead of one of the older versions?

    Thanks in advance!

    Have a nice day!


    Michael P 17 Dec at 13:10
  • Hi,

    I really like this model and it is very useful.

    The only thing I need to know is how can I use this for a blended inbound/outbound environment?

    Your help would be greatly appreciated.



    LiamG 20 Dec at 09:09
  • Why is it that the Erlang C calcuator only calculates for a half hour period instead of an hour?

    AL 14 Feb at 03:56
  • I historic call data for every 15 minutes. I’m predicting the calls based on a 90 percentile assumption.

    Can I change 1800 to 900 from cell g39 to g86 if I want to calculate agents required by 15 minute interval?

    Abbas Alwani 17 Feb at 17:22
  • If 90 contacts are expected in the next 1 hr. How many agents would be required if the AHT is 480 secs and occupancy is 80%.

    ulagu 22 Jul at 08:38
  • What %age of FTEs are more required if we move from 70-30 SL to 80-20 SL? Please help!

    Rohit 25 Aug at 12:10
  • The calculator will do that for you 🙂

    Change cells D18 and D21



    Dave Appleby 25 Aug at 12:54
  • Does the calculator consider by default an expected level of occupancy, or it has to be considered separately?

    Jerry 12 Sep at 04:16
  • Jerry,

    The calculator gives you the bottom
    line numbers you need.

    You have to factor shrinkage etc on top.



    Dave Appleby 12 Sep at 07:36
  • Is it unreasonable to expect valid forecasts for 85% in 300 seconds service level?

    Rich 26 Sep at 21:56
  • Hi,

    is it possible to convert this to hourly?specially the event planner. Can I just use hourly data for it?

    thank you

    ec 14 Oct at 09:47
  • Hi Can you tel me the difference between “Average call duration” and “Target answer time”? Thank you. Sincerely.

    Pierre 25 Jan at 19:11
  • Average Call Duration is the length of the telephone call – i.e. the time from when an agent started talking to the caller to the time when the call was ended. The Target Answer Time is how long that you target that the caller has to queue for – i.e. We wish to answer 90% of calls in 15 seconds.

    jonty pearce 26 Jan at 10:58
  • Thank you very much. Have a nice day.

    Pierre 27 Jan at 02:00
  • Hi

    I’ve read through all the comments and answers as well and still have a couple of questions relating to the calculator. It looks great though but before I can begin to use it for forecasting and resource planning can you answer the following please?

    1. Are the projected calls spread across the 30 minute slot equally or are they weighted to have more at the beginning of the half hour interval?

    2. With one of my service levels, we are only required to hit 50% SL. I have keyed in 50% as the required service level and the form calculates but is the predicted service level a percentage of the 50% or actual Service level that will be achieved?



    Matt 27 Jan at 22:19
  • Matt,

    To answer your questions.

    1) It looks at a balanced distribution during the arrival period, but…

    You can set the interval threshold to 900 sec instead of 1800 and work with the 15min intervals for greater granularity.

    2) Broadly speaking it’s what you can expect to achieve.

    Hope that helps.



    Dave Appleby 28 Jan at 13:05
  • When factoring Occupancy percentages, do you take Presented calls or handled in any given interval?

    Gregory Hall 5 Feb at 16:35
  • I am a big fan of your erlang calculator as the organization I work for does not have a workforce management suite with which to model call stats and staffing. I have used your tool for years to produce the Staffing Requirements (or other values)for cut and paste into Excel or Access tools. In the absence of a WFM software I have recently built a staffing model in Excel 2010 that requires I have multiple instances of the Erlang Calculator embedded in the multiple worksheets of the same workbook.

    The Calculator works perfect when I use it as a stand-alone excel workbook. If I copy the agentcalc worksheet from the stand-alone calculator and paste it into a new Excel 2010 worksheet, Excel no longer recognizes the erlang formulas for servicelevel, agentno, erlangc. The VBA shows, but it seems as though it has lost the connection to the .xla file.

    Do you have any recommendations on how or if I can do this.

    Paul Burns 21 Feb at 01:00
  • Hi Paul.

    Nice to see you find it useful!

    In order to duplicate it within a workbook you’ll need to follow a couple of steps.

    1) When you copy it over also copy the module from the VBA Window.

    2) servicelevel, agentno, erlangc are all named ranges you’ll need to change. I suggest just appending _1 (VBA will need the underscore)

    3) Change the variables in the new module (Anything that has a DIM statement at the top)
    again I suggest appending a _1 (same caveat applies)

    Give it a go, see what happens, and, if you get problems post them back here and I’ll try to help.

    Just as a suggestion have you thought about just calling a second workbook from the first and running
    the macro from the first book? That would eliminate the whole problem as you can then automate the whole cut and paste of the data…



    Dave Appleby 21 Feb at 11:17
  • Thanks for your quick response. I’ll report back after I try this. Kind Regards.

    Paul Burns 24 Feb at 02:18
  • In a call center environment, AHT is rarely consistent every 30 minutes. Do you plan to create a version of the Erlang Calculator where you can input the AHT forecast everyin 30 minute intervals along side the CV intervals?

    Anita 14 Mar at 15:48
  • I have AHT 240 , ASA 20, Calls 30 min interval as 40 , Target sl 80%, Now if i have to calculate req agent and forcasted sl , without using erlang what would be the step by step process

    Rohan 13 May at 21:55
  • I’m not sure why you cannot use Erlang, as this would be the easiest way to do it.

    The only other approaches are
    – Using Workforce Management software. There are lots of companies who do this in the directory.
    – Creating some form of simulation computer program. This would take a lot of time and effort.

    I’m not aware of any way to do it on paper.

    jonty pearce 14 May at 14:55
  • Its just a challange given to me and i really hope i could get soe help , pOisson probabilty migh help but i m unbale to make data to use Poisson , if some one can help with the data points

    Rohan 14 May at 18:36
  • The calculator works just fine – i plugged in my actuals and it is showing i should be doing fine while day on day or even on an interval basis I am failing. My assumption is that i have too much in office shrinkage which is not factored in this tool. I notice my Outbound activity and other auxes are taking away my staffing. how do i incorporate these to my planning for better coverage?

    Josh - philippines 17 Jul at 20:59
  • Do you have this for a weekly call profile calculation, this currently is a daily calculator.

    KC 28 Jul at 22:00
  • did you include the shrinkage in this calculator ?

    Anonymous 6 Aug at 13:12
  • I got this one in your Web :
    3. Does it allow for shrinkage?

    The short answer is no, it gives a required number of staff to be available on the phone for any given interval. Shrinkage is a bit too much of a local variable to be able to factor. Different companies use different criteria, personally I’ve seen:
    •1:1 sessions

    However, everyone has their own criteria.

    It can simply be corrected for by using: (FTE/Shrinkage %)*100

    WHERE we can apply this formula “by using: (FTE/Shrinkage %)*100” ??

    Quatiabh Allan 6 Aug at 13:17
  • Hi,
    My self Sudip.
    Now I have download Erlang sheet V3 it is brilliant thank you so much for providing this tools.
    Need help on this.
    I want to know how much occupancy included in this tools?
    Can I able to change the target of occupancy?


    Sudip 25 Aug at 11:18
  • Hi Dave, Thanks you for this tool!!! Worked with several WFM tools in the past, at my new job we don’t have one, and I really like this for the basic requirements. HAve one question, how can an interval with 3 calls have a service level of 95,45%?? I think it can only be 0%, 33,3%, 66,6% or 100% (0,1,2 or 3 calls within SLA). Can you please explain what I am missing?

    Peet Holland 7 Sep at 16:06
  • Can someone tell me what is “Required service level”. I am very new to this and i am in need of this kind of template

    Santhosh 20 Nov at 14:37
  • Hello, I was questioned whether this calculator has agent utilization built into the formulas and if we can adjust? I do not normally add utilization in my analysis, but am being asked to do so. I am unsure whether to include it in the Erlang-C calculation or in Shrinkage. Thanks!

    Liz Stilwell 26 Jan at 15:46
  • how to calculate number of staffs achieved between 80% to 95% using below data


    srinivas. S 24 May at 00:43
  • can this tool help in serious activities handled by the contact center and how. for example: WebChat, Emails and Calls

    Dana 6 Dec at 05:59
  • Hello, I need some help with Erlang c as it’s very new to me please.
    I know my av calls per hour/day/year, my handling and my FTE but I want to know how much extra FTE I will have for a year if I change my speed of answer from 2 minutes to 4 minutes. Do far ive only been able to see 2-5 FTE for an hour. Can you help please?

    alan 9 Dec at 18:32
  • Hi Alan
    If you can do it for an hour you can do it for a day. The long winded way to do would be to plot all of the calls per hour across the year.
    But if you do it for all of the hours in the day this will show you the total number of FTE needed for 2 minutes and also for 4 minutes. So if you needed an average of 15 agents to answer in 2 minutes and only needed 13 to answer in 4 minutes, then this would show a reduction of (15-13)/15 = 13%. You can then apply the 13% reduction figure to the number needed across a year.
    Obviously it would be more accurate if you could do it for a week.

    Jonty Pearce 12 Dec at 17:19
  • Jonty, you sir are a legend, thank you!

    alan 22 Dec at 15:26
  • I have not seen a reply to the question does the Erlang account for breaks/lunches? It appears when you calculate raw AHT and number of calls that there is some type of factor but is that more related to the service level and answer time?

    Michele 17 Jan at 15:59
  • Hi, can someone please help calculate the agents needed for the below 2 call types we receive in the 1 hour. The calculator only allows for the one call type but we have 2 varieties coming through. Many Thanks Mal

    Call 1
    248 calls in 60 minutes
    60 seconds AHT
    70%/20 sec service levels

    Call 2
    143 calls in 60 minutes
    150 seconds AHT
    70%/20 sec service levels

    Mal 27 Jan at 05:24
  • Does this work in Excel for Mac?

    Mark Kennedy 3 Feb at 23:19
  • We have not tested it but see no reason why it should not. Please let us know if you have problems.

    Jonty Pearce 9 Feb at 16:38
  • When downloading the Excel version of the Erlang-C calculator, it will not allow a time period greater than 545 minutes. I am trying to calculate # of agents required for a call center open from 8a-7p,(660 minutes) and the calculator keeps resulting with zero agents. It would also be nice if the download would allow you to enter AHT by interval instead of by day. Any assistance is greatly appreciated.

    Chad 7 Feb at 23:36
  • Thanks for pointing this out. We’ll get on to this and issue an update. Jonty

    Jonty Pearce 9 Feb at 16:33
  • Hi,

    Awesome calculator. Would this make sense to use for chat environment using 4 weeks previous NCH as this week’s volume? More importantly to factor in the concurrent chats is it a matter of dividing Agents Required by the expected number?

    Bilbo 14 Feb at 07:18
  • Erlang is not really the best solution for concurrent web chats, but in the absence of anything else it will at least give a rough estimate. The number that you divide by will need to be lower than the number of concurrent chats, as sometimes the agent will be only handling one chat. So if you had 3 concurrent chats then you would need to divide the number by say 2.5 instead of 3. You can then see how this compares with past patterns.

    Jonty Pearce 14 Feb at 15:03
  • Thanks Jonty!

    What are other solutions than erlang available to use for concurrent webchats?

    We know actual volume, ACD, SVL and shrinkage and are trying to determine needed agents per 15 minute interval.

    Bilbo 15 Feb at 07:56
  • Hi! Can anybody tell me, how is the definition of FTE? Is this 40 hours/week or what is the base there?
    Thanks for answering 🙂

    Matthias 16 Feb at 08:28
  • FTE = Full Time Equivalent. So is effectively the number of hours that a person works in a day or week.

    So 1 FTE = Somebody who works Monday – Friday
    or 1 FTE = Two people who work half a day

    In terms of the number of hours then an FTE could be either 37.5 hours or 40 hours, depending on the number of hours per week that you work in your organisation.

    Jonty Pearce 2 Mar at 13:46
  • Hello,
    I use the Erlang calculator. I would like to calculate the needed Number of Agents in Period of 09:00-21:00, 12 hours are 720 minutes. When enter 720 minutes it gives me 0 Agents?
    I use left calculator,
    I enter Incomimg calls 889, Average Handling Time 484 seconds,Requred Service Level 90,Target Answer time 60 seconds, Schrinkage 7 % or 30 %
    Where is my mistake?

    As Y 17 Feb at 10:10
  • There wa a bug in Version 5.1 for long durations. Please try again with Version 5.2. and let me know how you get on.

    Jonty Pearce 2 Mar at 13:35
  • Hi Folks,

    I need some assistance to calculate the expected idle time required for a certain SLA.
    How is it possible to get it with this tool or another one ?

    Thanks in adance

    Rémy 3 Mar at 16:33
  • Is there any type of Day Planner that takes into account Incoming Call Volume AND Average Handle Time by reporting interval?

    I love the day planner tab, however, if I want to accurately schedule wouldn’t it make sense to take into account fluctuating AHT by half hour (or 15 min) interval as well as call volume? It seems that a static AHT for the entire day wouldn’t be reliable.

    I’m looking for a less laborious way of determining required staff levels by 15 min interval than using the agent calculator for each interval of a day. In other words, my 16 hour operating window requires 64 calculations if I look at quarter hour intervals for each day of the week.

    Thanks in advance

    Jason 27 Mar at 23:52
  • It should be fairly easy to change the day planner to add in a column with different AHT figures per interval. Instead of using the $reference for AHT you can just link it to the cell on the row.

    Jonty Pearce 4 Apr at 16:41
  • Dear Team,

    I will really appreciate you, if you can provide the multi channel calculator or the calculation matrix in excel so that use it as per my requirement. basically i have 25 different skill sets and to improve the efficiency i just wanna do some calculation which is not possible through Erlang C therefore i am looking for simulation and your support.

    Yogesh 30 Mar at 11:48
  • I think that this requirement is beyond the capabilities of Excel. You will probably need a full blown Workforce Management (WFM) system to be able to do this.

    Jonty Pearce 4 Apr at 16:39
  • Hello, can you explain why a difference of +/- 1 agent can cause the probability to wait/prob. to answer in TT to flop from 0/100 or 100/0?

    Call Centre Helper Erlang Calculator Version 5.2
    Calculate probability of calls being answered or waiting:

    Sample data:
    Incoming calls 7100
    in a period of 60
    Average Handling Time 180
    Target answer time 20
    Number of agents 121
    Probability a call waits 0.00%
    Probability call is answered in target time (Service Level) 100.00%

    Incoming calls 7100
    in a period of 60
    Average Handling Time 180
    Target answer time 20
    Number of agents 120
    Probability a call waits 100.00%
    Probability call is answered in target time (Service Level) 0.00%


    Zach 30 Mar at 18:13
  • Hi,

    I used a previous version of this with success. I didn’t realise it had been updated. I tried to use the new version, but I seem to get unusual results.

    Based on AHT 516, Reporting period 60, Req SL 80%, Target Ans 20. Apparently I need 0 agents to handle 6 incoming calls. At 7 calls it tells me I need 3 analysts.

    Can you explain what might be the cause of this for low call volumes? On an hourly basis, I would expect agents required to be at least 1? I suspected rounding, but increasing the decimal places just showed 0.000000000 analysts.

    Many thanks in advance for your help.

    Joe 6 Jun at 18:52
  • Hi Zac and Joe we had just spotted those and have uploaded a new version which should solve those issues

    Rachael Boynton 7 Jun at 11:57
  • Hi,

    There is an error with Erlang Calculator. When I insert 7 avg calls per 30 minutes, it says I need 0 (zero) agents. When I insert I have 8 avg calls per 30 minutes, then it says I need 6 people. With a difference of only one call every 30 minutes, gives me a difference of 6 people. Also, the result could never be zero, as at least I would need one agent to answer the calls.
    I face the same issue with Day Planner, as in some parts of the day it says I need 0 (zero) people, when I would need at least one.

    Can you pls advise if you can fix this bug?

    Lucas Alves 9 Jun at 21:20
  • Pls disregard my comment I just left. I see the issue has been fixed and I already downloaded the new version.

    However there is another issue we need to look at on the Agentcalculator. If I insert 15 calls as avg every 30 minutes, it says I need 10 agents. If I insert 24 calls as avg (almost double) it says I need 11 people. Only one more person to handle almost 100% increase? Is this right?

    Lucas 9 Jun at 21:59
  • any comment on my query above?

    Lucas Alves 18 Jun at 16:22
  • downloaded the excel file and the day planner sheet populates data only for the first rows.
    I am trying to get it for the 24hours 15 minutes interval. Can anyone please advise if thats possible?

    Cecil 3 Aug at 16:44
  • Hi,

    Is there any way you could change the code within VBA in order to show FTE with decimals. If I have lines with very small call volume on interval I would be able to see whether I need 0.7 FTE or 1.4 etc. As it is now it only shows 0 or 1 for those small lines.

    Thanks in advance.

    Pat 8 Sep at 14:16