Erlang C Calculator
Enter the calls you expect in an interval, the average handle time and your service level target. The Erlang C calculator gives the agents you need on the phones, the probability a caller waits, the average speed of answer, occupancy and the headcount to schedule after shrinkage.
Call volume and handle time
Calls, or chats if each agent handles one at a time.
Talk + hold + after-call work, per call.
Service level target
Leave blank for no cap. 85% is a common ceiling.
Breaks, training, meetings, absence: paid time off the phones.
Result
Agents needed
14
On the phones, to meet the target
Scheduled headcount
20
After 30% shrinkage, rounded up
Traffic intensity
10
Erlangs: hours of calls per hour
Service level achieved
88.8%
Target 80% in 20 s
Probability of waiting
17.4%
Average speed of answer
7.8 s
Occupancy
71.4%
Answered with no wait: 82.6%
Agents vs service level
What each agent count gives for the same calls. Rows at or below the traffic cannot keep up: the queue keeps growing.
| Agents | Service level | ASA | Occupancy | P(wait) |
|---|---|---|---|---|
| 11 | 39.0% | 122.8 s | 90.9% | 68.2% |
| 12 | 64.0% | 40.4 s | 83.3% | 44.9% |
| 13 | 79.6% | 17.1 s | 76.9% | 28.5% |
| 14(your answer) | 88.8% | 7.8 s | 71.4% | 17.4% |
| 15 | 94.1% | 3.7 s | 66.7% | 10.2% |
| 16 | 97.1% | 1.7 s | 62.5% | 5.7% |
| 17 | 98.6% | 0.8 s | 58.8% | 3.1% |
| 18 | 99.3% | 0.4 s | 55.6% | 1.6% |
| 19 | 99.7% | 0.2 s | 52.6% | 0.8% |
| 20 | 99.9% | 0.1 s | 50.0% | 0.4% |
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How the Erlang C formula works
Start with traffic intensity in Erlangs: calls × average handle time ÷ the length of the interval. 100 calls of 180 seconds in 30 minutes is 18,000 ÷ 1,800 = 10 Erlangs, so ten agents would be busy all the time with no slack at all.
Erlang C then gives the probability that a caller has to wait for N agents, and from that the share answered within your target time and the average speed of answer. In Call Centre Helper's worked example the same 10 Erlangs with an 80% in 20 seconds target give 39.0% at 11 agents, 64.0% at 12, 79.6% at 13 and 88.8% at 14, so 14 agents meet the target, with a 7.8-second average speed of answer.
The calculator does this search for you and shows the agent counts around the answer, so you can see what one more or one fewer agent does to the service level.
Average handle time calculator
Average handle time (AHT) is talk time + hold time + after-call work, per call. From a report of totals: (total talk + total hold + total after-call work) ÷ calls handled. 240 seconds of talk, 30 on hold and 60 of wrap-up is an AHT of 330 seconds.
Open "Work out AHT from its parts" in the calculator to add the three up and drop the result into the staffing maths. Leaving after-call work out is the most common reason a staffing plan comes up short.
Occupancy rate
Occupancy is the share of logged-in time agents spend handling contacts: traffic ÷ agents. 10 Erlangs on 14 agents is 71.4%. Small teams run at lower occupancy for the same service level; large teams can run higher.
Very high occupancy leaves no time between calls, and many centres set a ceiling (Call Centre Helper's example uses 85%). Enter a maximum occupancy and the calculator raises the agent count when the cap, not the service level, is what decides it.
Shrinkage and the call center staffing number
Agents on the phones are not the people on the schedule. Breaks, training, coaching, meetings and absence take paid time away from the queue; that share is shrinkage. Scheduled headcount = agents ÷ (1 − shrinkage), rounded up: 14 agents at 30% shrinkage is 14 ÷ 0.7 = 20 people.
Work out each interval separately: a Monday 9:00 peak and a Thursday 15:00 lull need different numbers. For full-time equivalents across a week, use the FTE calculator.
What the model assumes
Erlang C assumes calls arrive at random (a Poisson process), every caller waits until answered, each agent handles one contact at a time and handle times average out. Real callers hang up, so Erlang C tends to overstaff when abandonment is high; models with abandonment exist for that case.
Chats fit only if each agent handles one at a time. Treat every result as a planning estimate and check it against what your queue actually did.
Results are planning estimates from the Erlang C model, not a forecast of your queue.
Worked example the calculator is checked against (read 12 October 2026):
For sales team leads: inside sales vs outside sales, cold call script, staffing sales.
Staffed for the calls. Now cut the wrap-up.
After-call work is part of every handle time. Talk a quick voice note into ParrotNotes as the call ends, or record a customer meeting, and get the summary and a follow-up email draft, so after-call work shrinks. Free for 100 minutes of recording a month.
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