Understanding & using Erlang

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CC Operations Design Specialist

Financial Services

Understanding & using Erlang
More of a post for Newbies really, and for those senior managers who are not familiar with this..

 Erlang is a formula for calculating the required number of agents to answer a given level of calls
 Erlang forms the basis of most Contact Centre resourcing software
 By using forecast call volumes, average handle times, and call compression, we can calculate resourcing for a variety of scenarios

Call Arrival
 Erlang accounts for the random arrival of calls within time intervals, and their synchronous nature – calls do not arrive at fixed intervals, and callers don’t wait until we have cleared a backlog
 If a Contact Centre receives calls in an unpredictable pattern (by the minute, hour or day), it will have busy and quiet periods

 If resourcing to meet call peaks, then agents are “idle” between peaks
 Idle time drives low occupancy
 Occupancy should aim for 80-85%
 86-90% occupancy is an empirical point where agents become stressed

Smoothing Effects
 Many contact centres have small contact teams c.10 FTE (50 FTE is a robust size)
 Doubling the call volume gives a smoother call profile, therefore combining teams reduces the total required number of FTe
 For example combined calls of 2 teams of 11 can be answered by 18

 In order to provide Erlang with a typical forecast call volume per half-hour the days calls are compressed into a block of core hours
 A typical compression for a 10 hours per day 5 days a week call centre would be monthly volume / 20 days / 8 hours
 The compression does not account for peaks and troughs, and this should be corrected using appropriate resourcing tools

Applying Shrinkage
 Once utilisation and shrinkage is known, this can be applied to the required FTE on the phones to give a figure for the total FTE needed to provide the required number
 By dividing the required FTE by the known shrinkage factor you obtain the paid FTE needed

Fig. 2.2 Erlang example

Service Level 75% of calls in 10 seconds
Call arrival per hour 100
Average call handle time 180 seconds
Average wrap time 30 seconds
Required FTE 9 FTe
Paid FTE @ 85% utilisation 11 FTe

Service Level 75% of calls in 10 seconds
Call arrival per hour 200
Average call handle time 180 seconds
Average wrap time 30 seconds
Required FTE 15 FTe
Paid FTE @ 85% utilisation 18 FTe

Project Manager and Consultant

TCC Global Limited

Have You Seen Elitium
You appear to be completely at ease working manually with Erlangs within a Call Centre domain. Have you come across any modelling products other than Elitium which allows you to drill down to the process level and incorporates Erlangs within the embedded calculations. I'm not punting Elitium to you but we are always on the look out for competitive products. There are some out there that do similar things but there does not appear to be any that are as comprehensive. What do you use at ZFS.

Please don't be offended by my approach.

many thanks,


Call Centre Helper

The Erlang C Formula
We have just written an article with a worked example of the Erlang C Formula that should explains how it works.


Erlang - does it indicate total number of Agents required for the day
Does the erlang calculator (version 5.5) show how many Call Centre Agents you need in total to manage a Call Centre comfortably or does it indicate how many Call Centre Agents are required per day in a Call Centre?


Call Centre Helper

Use the online Version
If you can it is better to use the online version. This looks at Shrinkage as well as Maximum Occupancy. It also has a day planner to show you how the staffing patterns change across the business day.


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