Erlang C Calculator (Call Center Staffing)
Work out how many call center agents you need with Erlang C: service level, average speed of answer, occupancy and shrinkage, with a table for each staffing level.
Erlang C assumes callers wait as long as needed (no abandonment) and calls arrive at random, so it slightly overstates the agents needed when many callers hang up.
| Agents | Probability of waiting % | Service level % | ASA (s) | Occupancy % |
|---|---|---|---|---|
| 11 | 68.21 | 38.96 | 122.8 | 90.9 |
| 12 | 44.94 | 64.02 | 40.4 | 83.3 |
| 13 | 28.53 | 79.56 | 17.1 | 76.9 |
| 14 | 17.41 | 88.84 | 7.8 | 71.4 |
| 15 | 10.2 | 94.15 | 3.7 | 66.7 |
| 16 | 5.73 | 97.06 | 1.7 | 62.5 |
| 17 | 3.09 | 98.58 | 0.8 | 58.8 |
Erlang C is the standard formula for call center staffing. From the number of calls, how long each one takes and the service level you promise (such as 80% of calls answered within 20 seconds), it finds the smallest number of agents that meets the target. This calculator also checks occupancy, so agents are not run at 95% busy, and adds shrinkage to turn agents on the phones into agents to schedule.
How to use this tool
- Enter the calls expected in one interval and pick its length (15, 30 or 60 minutes).
- Enter the average handle time in seconds, including hold and wrap-up.
- Set the service level target, the maximum occupancy and your shrinkage.
- Read the agents needed and compare neighbouring staffing levels in the table.
Formula
A = calls × AHT ÷ interval; C(n, A) = n·B ÷ (n − A(1 − B)); SL = 1 − C·e^(−(n − A)·t ÷ AHT); ASA = C·AHT ÷ (n − A)
A is the traffic in Erlangs and n the number of agents. B is the Erlang B blocking probability, computed with the stable recursion B₀ = 1, Bₖ = A·Bₖ₋₁ ÷ (k + A·Bₖ₋₁). C is the probability that a caller has to wait, t the answer-time target and occupancy = A ÷ n. Scheduled agents = n ÷ (1 − shrinkage).
Example
100 calls in 30 minutes with a 180-second handle time is 100 × 180 ÷ 1,800 = 10 Erlangs. With 13 agents the service level is 79.6% — just short of 80/20. With 14 agents it is 88.8%, 17.4% of callers wait, the average speed of answer is 7.8 seconds and occupancy is 71.4%. With 30% shrinkage you schedule 14 ÷ 0.7 = 20 agents.
Frequently asked questions
What is a good occupancy?
Around 75–85% for inbound calls is common. Above that, agents get no breathing space between calls and burn out; very large teams can run higher because their traffic is smoother.
Why do small teams need relatively more agents?
Random arrivals bunch up. With 180-second calls, 10 Erlangs of traffic needs 14 agents (40% spare) for 80/20, but 100 Erlangs meets the same target with 107 agents (7% spare). This economy of scale is built into the Erlang C formula; at that size an 85% occupancy cap (118 agents), not the service level, sets the number.
Does Erlang C work for chat or email?
Not directly. Chat agents handle several sessions at once and email has no answer-time pressure, so the queueing assumptions do not hold. Use it for live voice queues.
Sources
Last updated:
Page address with these inputs: https://kakao.mobile/tools/erlang-c-tool/
Cite or embed this tool
kakaoCalc. (2026, November 8). Erlang C Calculator (Call Center Staffing). https://kakao.mobile/tools/erlang-c-tool/
“Erlang C Calculator (Call Center Staffing).” kakaoCalc, 8 Nov. 2026, kakao.mobile/tools/erlang-c-tool/.
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