Erlang calculator for contact center staffing

Method

How the calculator works

Step one is traffic in Erlangs: calls per interval × AHT ÷ interval length. If 120 calls of 4 minutes each arrive in an hour, traffic is 8 Erlangs. That's how many agents would be busy nonstop if calls came in perfectly one after another.

In reality, calls arrive unevenly, and eight agents won't be enough: the queue builds up during spikes. So the calculator tries one option after another – 9 agents, 10, 11 – and for each uses the Erlang formula to work out what share of callers will wait and how many get answered within your target time. Once the service level target is met and occupancy stays under the limit, that's the number.

Headcount is that number divided by the share of time agents actually spend on the line. It's rounded up: you can't schedule half a person.

Erlang A mode is closer to real life: callers don't wait forever, and some hang up. You enter how long a caller is willing to wait on average in the Average caller patience field. Here, service level is the share of all calls answered on time, and abandoned calls don't count toward it. It usually takes fewer agents than Erlang C, and you see right away how many calls you lose.

We checked the math against a reference example: 100 calls in 30 minutes at 3 minutes each, a target of 80% answered in 20 seconds, occupancy capped at 85%, and 30% shrinkage. The calculator gives 14 agents on the line, an 88.8% service level, a 7.8-second average speed of answer, and a headcount of 20 – matching the reference.

Result

How to read the result

• Agents on the line – how many people need to be in their seats and ready to take calls during this interval.
• Headcount after shrinkage – how many people to schedule so that, after breaks, training, and PTO, you still have the right number on the line.
• Calculated service level – the share of calls answered within your target time. It can come out above target, since you can't staff a fraction of an agent.
• Average speed of answer – only an average: one caller waits a second, another a minute. That's why it's better to set your target as a service level.
• Calls that wait in queue – how many callers don't reach an agent right away.
• Abandoned calls – how many callers hang up before they get an answer. Shown only in Erlang A mode.
• Agent occupancy – the share of time an agent is busy on a call. Above 85-90%, there's almost no breathing room, and people burn out fast.
• The Plan shifts button below the result passes the number of agents and shrinkage to the shift calculator, where they turn into headcount and a schedule.

Example

Example

A support line takes 120 calls in its peak hour, and each takes 4 minutes including after-call work. To hit 80/20, you need 11 agents on the line. If 30% of time goes to breaks and absences, you need to schedule 16 people for that hour.