Erlang A calculator
Erlang C assumes nobody ever hangs up, so it overstates queues when staffing is tight. Erlang A adds the average time a customer will wait before abandoning, and predicts the abandonment rate as well as the service level. Enter the contacts, handle time, patience and targets, and the calculator finds the smallest number of agents that meets your service level and your abandonment ceiling.
Meets both the service level and the abandonment ceiling, before shrinkage.
- Service level
- 89.5%
- Abandoned
- 4.2%
- Average speed of answer
- 3.3 s
- Contacts that will wait
- 21.4%
- Occupancy
- 86.3%
- Erlang C would need
- 39
- Workload
- 33.3 erlangs
Answered within target ÷ offered.
Answered contacts only.
Same target, no abandonment.
Show the working
- Workload = 200 contacts ÷ 30 min × 300 s ÷ 60 = 33.3 erlangs.
- Erlang A is the Erlang C queue with every waiting customer hanging up at a rate of 1 ÷ 90 s. Because customers leave, the queue is stable even with fewer agents than erlangs.
- With 37 agents: 21.4% of contacts wait at all, 4.2% abandon, and 89.5% are answered within 20 s. That clears the 80% service level and the 5% abandonment ceiling.
- Carried load is 33.3 × (1 − 0.042) = 31.9 erlangs, so occupancy is 86.3%.
- Erlang C alone, which assumes infinite patience, would need 39 agents for the same service level.
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How the Erlang A calculation works
Erlang A is Erlang C with one extra ingredient: each customer in the queue has a patience, and hangs up when it runs out. Patience is modelled as exponential with the mean you enter, which is the same shape assumed for handle time and is the standard choice.
Because impatient customers leave, the queue never grows without limit, even with fewer agents than erlangs of work. The model predicts three things Erlang C cannot: the share of contacts that abandon, the wait experienced by those who are answered, and a service level that counts abandoned contacts as not answered, which is how an ACD reports it.
The calculator increases the agent count until both the service level target and the abandonment ceiling are met. For a well-staffed queue the answer is close to Erlang C's; the two diverge when staffing is tight, where Erlang C predicts long queues that in practice turn into abandoned calls.
Read the full guide: Erlang A: staffing when customers hang up
Frequently asked questions
- How do I estimate average patience?
- Take the average time to abandon from your ACD, but only for contacts that abandoned after any short-abandon threshold. It is a biased estimate, because it only sees customers whose patience ran out before they were answered, and it understates true patience when service is good. Use it as a starting point and check the model's predicted abandonment against what actually happened.
- Why does Erlang A need fewer agents than Erlang C?
- Contacts that abandon do not consume an agent, so the carried load is lower than the offered load, and the customers still waiting see a shorter queue. Erlang C counts every one of those as a long wait that never happens. The difference is small when staffing is generous and large when it is not.
- Should I plan with Erlang A or Erlang C?
- Plan with Erlang C for the requirement and use Erlang A to understand what happens when you fall short: how many customers you will lose and what the answered ones will experience. Staffing to Erlang A alone bakes an abandonment rate into the plan, which is only right if the business has agreed to it.
- What is the difference between the service level here and in the Erlang C calculator?
- Here it is contacts answered within the target divided by contacts offered, so an abandoned contact counts against it. Erlang C has no abandons, so its figure is the share whose wait is under the target.
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