In a call centre with a reasonably consistent call flow (no large, irregular peaks or troughs), we can make a very helpful observation:  What goes on in an “average hour” is generally representative of a centre overall.

For example, consider a call centre which is offered 20,000 calls per week, opens 08:00-20:00 weekdays and 08:00-18:00 at weekends.  The AHT for the week is 330 seconds, and a performance target of answering 80% of calls in 30 seconds has been set.

As it is open for 80 hours per week, the “average hour” sees 250 calls.  The accountant thinks you need 250 x 330 / 3600 = 22.9 advisors in this hour; Erlang (using our spreadsheet add-in) thinks we need 28 advisors to answer 80% of calls in 30 seconds.

In other words, to deliver the right service level, you need to employ 22% more people than the accountant said you would.

We can use this logic as part of a figure for the call centre as a whole:  the accountant says we need 20,000 x 330 / 3600 = 1,833 call-taking hours.  We can uplift this to say we need 1,833 x 122% = 2,240 call-taking hours to deliver the required service level.