About the Erlang C Call Center Staffing Calculator
This Erlang C calculator works out how many agents a contact centre needs on the phones in an interval to meet a service level target such as 80% of calls answered within 20 seconds. Enter the number of calls you forecast for the interval, the average handle time (talk + hold + after-call work) and your target, and it returns the minimum number of agents, the service level and average speed of answer (ASA) that staffing delivers, and how busy agents will be.
It is meant for workforce planners, team leaders and operations managers building interval staffing plans, testing what a change in volume or handle time does to headcount, or explaining to finance why one extra agent can move service level so much. The table shows service level, ASA and occupancy for a range of agent counts, so you can see the trade-off directly.
Erlang C assumes calls arrive randomly (Poisson), handle times are exponentially distributed, nobody hangs up while waiting and every call that waits is eventually answered. Because real callers abandon, Erlang C tends to slightly overstate the agents needed, which most planners treat as a safety margin. Shrinkage (breaks, training, absence, meetings) is then added on top to get the number of staff you must schedule.
With the default inputs, the agents required on the phones is 32. Change any value above to recalculate instantly.
How to use the erlang c call center staffing calculator
- 1Enter the forecast number of calls for one interval and choose the interval length.
- 2Enter the average handle time in seconds, including hold and after-call work.
- 3Set your service level goal, e.g. 80% of calls answered in 20 seconds.
- 4Optionally cap occupancy so agents are not run at 95–100% busy.
- 5Add your shrinkage percentage to convert agents on the phones into staff to schedule.
- 6Use the table to see how service level and ASA change with one more or one fewer agent.
Formula and method
Traffic intensity A (in Erlangs) is the workload: calls in the interval × average handle time ÷ interval length in seconds. The Erlang C formula gives Pw, the probability that an arriving call has to queue when N agents are logged in. The calculator evaluates it with the numerically stable Erlang B recursion, B(k) = A·B(k−1) ÷ (k + A·B(k−1)), then C = N·B ÷ (N − A(1 − B)), which avoids overflow from large factorials.
Service level is the share of calls answered within t seconds, 1 − Pw × e^(−(N−A)t/AHT), and the average speed of answer is Pw × AHT ÷ (N − A). The calculator starts at the smallest N above A and adds agents until both the service level target and the optional occupancy cap are met. Staff to schedule = agents ÷ (1 − shrinkage).
- λ
- Calls offered in the interval
- AHT
- Average handle time in seconds
- T
- Interval length in seconds
- A
- Traffic intensity in Erlangs
- N
- Agents on the phones
- Pw
- Probability a call waits (Erlang C)
- t
- Target answer time in seconds
Worked examples
200 calls per half hour, 4-minute AHT, 80/20
200 calls × 240 seconds ÷ 1,800 seconds = 26.67 Erlangs. With 31 agents only about 77.7% of calls are answered within 20 seconds, so 32 agents are needed: they reach about 84.9% with an ASA of roughly 10.6 seconds and 83.3% occupancy. Allowing 30% shrinkage, about 45.7 people must be scheduled.
Small team: 60 calls per hour, 5-minute AHT, 90/30
60 calls × 300 s ÷ 3,600 s = 5 Erlangs. Eight agents reach only about 87.6% within 30 seconds, so 9 agents are needed, giving about 94.6% service level at 55.6% occupancy. With 25% shrinkage that is 9 ÷ 0.75 = 12 people scheduled. Small teams need proportionally more spare capacity than large ones to hit the same target.
Occupancy capped at 80%
The default scenario meets 80/20 with 32 agents at 83.3% occupancy. Capping occupancy at 80% means at least 26.67 ÷ 0.8 = 33.3 agents, so 34 are required, which also lifts service level to about 93.4%.
Frequently asked questions
What is Erlang C used for?+
Erlang C is the standard queueing formula for sizing inbound call centres. It estimates the probability a caller waits, the service level and the average speed of answer for a given number of agents, so planners can find the fewest agents that meet a target.
What does 80/20 service level mean?+
80/20 means 80% of calls are answered within 20 seconds. It is a widely used default target, but many centres use other targets such as 90/30 or 70/60 depending on the channel and customer expectations.
Why does Erlang C overestimate staff?+
Erlang C assumes nobody abandons the queue. In reality some callers hang up, which removes workload and shortens queues, so actual service level is usually a little better than predicted. Erlang A models abandonment if you need more precision.
What is shrinkage in call centre staffing?+
Shrinkage is the share of paid time agents are not available to take calls — breaks, lunch, coaching, training, meetings, sickness and holidays. If shrinkage is 30%, you must schedule agents ÷ 0.7 people to have the required number on the phones.
What is a good agent occupancy rate?+
Occupancy is the share of logged-in time agents spend handling calls. Many operations aim for roughly 80–90%; sustained occupancy near 100% means no recovery time between calls and tends to increase burnout and attrition.
Why do small teams need relatively more agents?+
Queues are less efficient at low volume: random bursts of calls have a bigger effect when few agents are available. Larger teams pool demand better, so they can run at higher occupancy for the same service level.