Free tool
Call center staffing calculator (Erlang C)
How many agents do you need on the phones to answer your calls within your target? Enter your own call volume, handle time and service-level goal; the calculator applies the standard Erlang C formula.
| Agents | Service level | Chance of waiting | Avg. speed of answer | Occupancy |
|---|---|---|---|---|
| 12 | 64% | 44.9% | 40.4 s | 83.3% |
| 13 | 79.6% | 28.5% | 17.1 s | 76.9% |
| 14 (meets your target) | 88.8% | 17.4% | 7.8 s | 71.4% |
| 15 | 94.1% | 10.2% | 3.7 s | 66.7% |
| 16 | 97.1% | 5.7% | 1.7 s | 62.5% |
| 17 | 98.6% | 3.1% | 0.8 s | 58.8% |
How it is calculated
- Traffic A (Erlangs) = calls × average handle time ÷ interval length in seconds
- Chance of waiting C(N, A) = Erlang C formula for N agents (requires N > A)
- Service level = 1 − C(N, A) × e−(N − A) × answer time ÷ handle time
- Average speed of answer = C(N, A) × handle time ÷ (N − A)
- Occupancy = A ÷ N
- Agents = the smallest N whose service level meets your target
- Scheduled staff = agents ÷ (1 − shrinkage), rounded up
Planning rules and limits
- Calls are assumed to arrive at random and nobody hangs up while waiting, so the result usually errs toward more agents.
- One queue, identical agents. Multi-skill routing, chat and callbacks are not modelled.
- Run it for each interval; staff the busiest one, not the daily average.
- Callers waiting in queue also use phone lines: plan line or channel capacity for agents plus the queue you allow.
- High occupancy leaves little slack; review breaks and burnout risk with your team.
Planning estimate only. OttoCall's contact center offers queues, skill-based routing and live dashboards on Premium and above — see the call center KPIs guide for the metrics behind this calculator.
Questions about this calculator
- What is Erlang C?
- A queueing formula, published by A. K. Erlang, that estimates the chance a caller has to wait when a given number of agents answer randomly arriving calls. From it you get service level, average speed of answer and occupancy for each staffing level.
- What does 80/20 service level mean?
- 80% of calls answered within 20 seconds. Enter your own target percentage and answer time; the calculator finds the smallest number of agents that meets both.
- What is shrinkage?
- The share of paid time agents are not available to take calls — breaks, training, meetings, absence. Scheduled staff = agents needed ÷ (1 − shrinkage), rounded up.
- Is the result exact?
- No. Erlang C assumes callers never hang up while waiting and that calls arrive at random, so it tends to suggest slightly more agents than a queue with abandonment needs. Treat the result as a planning starting point and check it against your own data.