FTE Calculations

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Infinity Communications

FTE\'s in a Call Centre
I'm required to calculate the number of FTEs in a Call Centre.
What I have is:

Number of Inbound Calls per Month is 1,320,000:

AHT is 120 Seconds

Service Level 85%

1 FTE is 172 Hours per month.


Call Centre Helper

You need the Erlang Calculator
You need to break down the calls by half hour and then you need to use a call centre calculator. (This is often known as an Erlang calculator).


WFM Analyst

Hi Stells,

I'm new to workforce, this will be what i will do according to the available information. Will be more accurate if plan shrinkages and occupancy can be provided as well.

Volume 1320000

AHT 120

Total 158400000

Total Hours 44000

FTE hours / Month 172

Total FTE Needed 256

WFM Analyst

Manual vs Erlang C
I'm new to workforce, this will be what I will do according to the available information. It will be more accurate if plan shrinkage and occupancy as well.
Volume: 1320000
AHT: 120
Total: 158400000
Total Hours: 44000
FTE hours per Month: 172
Total FTE Needed: 256

I made some calculations for Manual VS ErLang C (which consider SLA & Waiting time).
Below are my findings. The result of 256 FTE was the manual calculation without consider SLA & waiting time.

I give a default 20 second wait time allowance. Below will be the SLA target and how much of additional FTE needed compare with manual. 85/20 SLA will need additional 58% FTE.

100 481%

90% 67%

80% 51%

70% 41%

60% 33%

50% 27%
Please share your thought.


Call Centre Helper

Hi Ivan

The manual calculations are usually wrong as they do not factor in the call arrival patterns and assume that the call volume is even across the day and that there is no service level calculation.

I would always take the Erlang output over a manual calculation.

WFM Analyst

Manual Calculations
Yes, agree with you advice. But I do have a concern that in a situation where the business is unable to provide volumes in half hourly formats that I can only use the manual calculation method that include shrinkage and occupancy.

And if I just average the volume to 48 intervals in 24 hours, the FTE required will not be accurate due to call arrival pattern as you advise.

Therefore, I would like to find out how to make the manual formula calculation get a result closer with Erlang C (with SLA consideration).
Thus, I did some research and some trial and error which gave me: For 90/20 I will need to add in additional 67% FTE in the manual calculation or 80/20 will be addition 51%.

FTE required
Please help me out as I need your assistance regarding FTE requirements:

Expected call volumes: average volume of 74,000 per month

Service Level : 75% within 60 seconds

AHT: 180 seconds

Support hours: 9am to 2am = 17 Hours

How Many FTEs do I require ?


Call Centre Helper

The best way to do this is through Web Chat

The best way to do this is using an Erlang Calculations.

You first need to split your calls across a week and then by hour to match your profile. You then use the Erlang spreadsheet to calculate the raw number of people you need every hour.

There is a spreadsheet here


There is also an online version here


7 day operation with 5 day working weeks
Thank you so much Jonty. That was really helpful.

If its a 7 day operation, how many FTE do we require?

All agents will be working 5 days per week. Thank you so much for your help.


Call Centre Helper

Look at your traffic profile
It tends to depend upon your traffic profile.

I tend to find that on average 22% of the volume comes in on a Monday and that tails down to 17% by Friday. You will also need to work out your daily call arrival profile. You can probably get this off your current phone system.

You will also need to build in a "shrinkage factor" typically around 50-70% to allow for time off phones such as meetings, holiday, sickness etc.

Required FTE
CALLS*AHT = 1320000*120 = 158400000 = WORKLOAD
CONVERT WORKLOAD INTO HOUR = 158400000/3600 = 44000
FTE = 44000/172 = 255.814 = 256(APPX.)


Call Centre Helper

Your required FTE is wrong - use Erlang, shrinkage and max Occupancy to get the right figures

You have just put in the traffic intensity to get your answer of 256 FTE

You have not included these three factors
- Service Level
- Shrinkage
- Maximum Occupancy

You need to use an Erlang Calculator to work this out


1,320,000 calls per month = 303,448 calls per week.

You normally get around 23% of call volume on a Monday so this works out as 63724 calls per day.

Putting this into the Erlang Calculator shows that you would need a peak of 441 agents, which is significantly higher than the 256 agents you have suggested.

Required FTE
Is this FTE calculation correct?

17000 Calls 120 Seconds handling time with 8 login hours in a day


60 Minutes*60 seconds= 3600




Call Centre Helper

You have to break it down by interval

What you have worked out is the average traffic intensity. Unfortunately the average is not of much help.

You need to put this into an Erlang calculator

So Assuming

  • 17000 calls per 8 hours

  • AHT Time 120 seconds

  • 80 % Answered in 20 seconds

  • Shrinkage 30 %

  • Max Occupancy 85 %

This with a typical traffic profile gives as peak of 1312 calls per half hour.

Maximum Calls per Period: 1316
Maximum Number of Agents: 148.5
Average Number of Agents: 127.8
Average Occupancy: 79.2%
Service Level for Day: 99%
Answered in 20 Seconds
Average Speed of Answer (ASA): 1 Seconds
Answered Immediately:91.4%

This is much higher than the 70.8 advisors that you suggest.

You must factor in

  • Traffic broken down by interval (typically per half hour)

  • Shrinkage

  • Maximum occupancy

Try for yourself on this link

how to apply 80/20 SL manually without using the Erlang calculator.
Daily volume = 1410 calls
AHT = 3.11 mins
Daily Agent working hours =8
Weekly Agent working days =5 days
Agent working hours in mins = 8*60 = 480 mins
Daily Interactions handled by an agent = 480/3.11 = 96 calls
Interactions handled by agent with 85% occupancy = 96*85/100 = 82 calls
Daily agents required = 1410/82 = 11 agents
Daily agents required with 30% shrinkage = 11*30/100 =14 agents
Weekly required agents = 14*5/7 = 20 agents
A total of 20 FTE are required to handle 1410 calls.

Please confirm the above calculation method is correct and advise how to apply 80/20 SL manually without using the Erlang calculator.


Call Centre Helper

You need to use an Erlang Calculator
It is not possible to do this without an Erlang Calculator


How to Calculate Number of Agents Required for Emails in a Contact Center?
Can I apply the Erlang Calculator to calculate how many agents I need to complete incoming emails in a contact center? For example, we are to receive 1200 emails in a period of 24 hours, we work a clean 8 hours in a day and our average responses per hour targeted is about 6.3 emails. We are targeting a SL of a minimum 85%. If the Erlang Calculator is not suitable, is there any other recommended tool? Thank you very much in advance!


Call Centre Helper

Erlang Calculator can also do emails
Yes you can also use the Erlang Calculator to work out emails as well

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