How to calculate telephone capacity

There are a number of resources which practices could use to calculate the capacity required to ensure good telephone response. The following Links direct you to these resources.

These are external links which the Foundation has no control or influence on.

The Danish mathematician Agner Krarup Erlang – an expert in telephone systems – devised the standard formula used for calculating the maximum number of calls per hour that can be handled by a given number of agents (receptionists/call-handlers) given an average call length and the service level that is desired.  We have provided below some links to resources that are available on the web.  We do not provide any guarantee or recommendation for any of the sites and you will find others by searching for Erlang calculators, but we hope that these are useful to you.

Link 1.  Department of Mathematics, Amsterdam University.  This takes you to an on-line calculator that will calculate the service level.  Fill in the average call volume for an hour (arrivals), the average length of the call (service time) and the number of staff devoted to answering the phones (number of agents).  We also recommend that you fill in the bottom right box with 30 seconds so as to calculate the percentage of calls that will be answered in 30 seconds – as a guide the National standard for out of hours services is that 95% of calls should be answered in 30 seconds, a value below 90% in 30 seconds is generally thought unacceptable by telephone call-centre experts.

Link 2 and 3.  Free add-ins for excel. This add-in will allow the calculation of service level given a number of agents or the number of agents required to deliver a particular service level and you can build the formulae into an excel model.  Alternatively there is a free calculator provided by call centre at

Link 4.  There is a freeware version of cc-modeler professional known as cc-modeler lite at http:www.kooltoolz.comccm.htmgclid=CJbU1vm8vZQCFQKaFQod1ChLTg
Again this will allow the calculation of service level given a certain number of agents or the calculation of the number of agents needed to deliver a particular service level.

Link 5.  For the mathematically minded the formulae are provided at:  Here they also describe how the various formulae can be written into excel.
Link 6.  Wikipedia provides an overall description of the Erlang formula and what it does at:

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