Work out your staffing needs with the Erlang C calculator.
Enter call volume, handling time and service level – the calculator shows how many people must be on the phone at the same time. It runs entirely in your browser; your inputs are not transmitted.
| Staff at the same time | Service level | Average wait | Occupancy |
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Calculated with Erlang C: workload (Erlang) = calls per second × handling time. The service level follows from the probability of waiting and the time threshold. A planning value without guarantee; it does not replace detailed planning with your interval data.
This is how the calculator works.
Erlang C is the standard model for staffing inbound telephone queues. You enter how many calls arrive in an interval, how long a call takes on average including after-call work, and which service level you want to reach – for example 80% of calls answered within 20 seconds. The calculator returns the number of people who must be available at the same time, the service level actually reached, the average waiting time and the occupancy.
The table shows what happens with one or two people more or fewer. It makes the most important property of queues visible: close to the limit, a single person changes the service level considerably.
This is how to read the result.
- Staff at the same time: the number of people who must be logged in and available during this interval – not the head count on the rota.
- With shrinkage: the number of people you need to schedule so that enough are actually available after breaks, training, illness and holidays.
- Occupancy: the share of time staff spend on calls. Values that stay very high for long periods increase strain and error rates.
- Per interval: calculate each half hour or hour separately. A daily average hides the peaks that decide whether callers get through.
The model has limits you should know.
Erlang C assumes that calls arrive randomly, that nobody hangs up while waiting and that every member of staff can handle every call. In practice, some callers abandon, which lowers the real requirement slightly. With skill-based routing, several languages or chat with parallel conversations, the model no longer fits exactly. Use the result as a sound order of magnitude and refine it with your own interval data.
If your volume fluctuates strongly, an overflow model can cover the peaks without staffing for the maximum all day. More terms are explained in our glossary.
Questions about the Erlang C calculator.
Question not listed? We reply within one working day.
Erlang C estimates how many people must be available at the same time so that a given share of calls is answered within a given waiting time. It uses the call volume per interval, the average handling time and the target service level.
Calls arrive randomly, not evenly. To answer most of them quickly, some staff must be free at any moment. That buffer is why occupancy stays below 100 percent and why small teams need proportionally more buffer than large ones.
Shrinkage is paid time in which staff are not available for contacts: breaks, training, meetings, illness and holidays. The calculator divides the required staff by (1 − shrinkage) to show how many people you need on the rota.
It is a planning model. It assumes that nobody hangs up while waiting and that all staff can handle all calls, so it tends to overestimate slightly. For multi-skill routing, chat with parallel conversations or strong peaks, a simulation or your workforce management tool is more precise.
