Erlang C — voice staffing calculator
How many agents does your inbound call queue need? Enter your volume, handle time, and service level target. Results update as you move the sliders.
Staffing requirement
Predicted performance
How this calculator works
What is Erlang C?
Erlang C is a queueing theory formula that models an inbound call center as an M/M/N queue: Poisson call arrivals, exponentially distributed handle times, and N agents sharing a single queue. It calculates the probability that an arriving call must wait, which lets you determine the minimum number of agents needed to hit a target like “80% of calls answered within 20 seconds.”
When should you not use it?
Erlang C assumes no abandonment, steady demand within the interval, and a single skill group. For email, tickets, back-office cases, or asynchronous messaging it will give you the wrong answer — those need a backlog and productive-hours model. For live chat with concurrent handling, you need a concurrency-adjusted model.
What is shrinkage?
Shrinkage is the fraction of paid working time during which an agent is not available to take calls: breaks, team meetings, training, coaching, admin tasks, sick leave, and unplanned absence. A 25% shrinkage means that for every 4 agents scheduled, only 3 are seated and available. The calculator converts seated requirement to scheduled headcount automatically.
What is the occupancy guardrail?
Occupancy is the fraction of seated time agents spend actively handling calls. At 95%+ occupancy, agents never get a break between calls — quality drops, stress rises, and attrition follows. This calculator refuses to recommend staffing above your occupancy guardrail (default 85%), even if the service level math would technically allow it.
Planning email, chat, or tickets too?
Different channels need different math. Erlang C is built for real-time voice queues. For other channel types:
- Email / tickets / back office: use the email & ticket calculator — it models productive hours, backlog, and SLA deadlines.
- Live chat: use the live chat calculator — agents handle multiple sessions simultaneously, so it uses a concurrency model, not Erlang C.
- Asynchronous messaging: treated as a backlog/response-SLA problem, same model as email. Use the email calculator.
Need forecasts, schedules, and cost tracking?
The free calculator answers a single staffing question. Turnella connects it to a forecast, a shift schedule, and a labour cost model that updates as your data changes.