Service level and ASA calculator
The Erlang C calculator asks how many agents you need. This one asks the reverse: with the agents you have, what service level and average speed of answer will you deliver? Enter the contacts, handle time and agents for an interval and it predicts the service level at your target answer time, the ASA, the probability a contact waits at all, and the occupancy.
Contacts answered within 20 seconds.
- Average speed of answer
- 21.4 s
- Contacts that will wait
- 33.3%
- 90% answered within
- 77.4 s
- Average wait of those who wait
- 64.3 s
- Occupancy
- 87.7%
- Workload
- 33.3 erlangs
The 90th percentile of the wait.
Show the working
- Workload = 200 contacts ÷ 30 min × 300 s ÷ 60 = 33.3 erlangs.
- With 38 agents, Erlang C gives a 33.3% chance that a new contact finds every agent busy.
- Service level = 1 − 0.333 × e^(−(38 − 33.3) × 20 ÷ 300) = 75.6%.
- Average speed of answer = 0.333 × 300 s ÷ (38 − 33.3) = 21.4 s across all contacts; the 33.3% who wait average 64.3 s.
- Occupancy = 33.3 ÷ 38 = 87.7%.
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How service level and ASA are calculated
The workload is contacts per minute multiplied by handle time in minutes, in erlangs. Given that and the agents staffed, Erlang C gives the probability that a new contact finds everyone busy and joins the queue.
Service level is the share of contacts answered within the target time. Contacts that do not wait count as answered immediately; for those that do, the chance of still waiting falls away exponentially at a rate set by how many spare agents there are relative to handle time, which is where the target answer time comes in.
Average speed of answer averages over every contact, including those answered immediately, so it is usually much lower than the average wait for those who queue. Both are averages across a whole interval; a bad ten minutes inside it can look worse on the floor.
Read the full guide: How to calculate service level and average speed of answer
Frequently asked questions
- What does an 80/20 service level mean?
- 80 per cent of contacts answered within 20 seconds. The two numbers go together: 80/20 and 90/60 are different targets and can need different staffing. Use the target the business is actually held to, and the same definition (from queue entry, after the IVR) that your ACD reports.
- Is ASA a good target?
- ASA is easy to explain but hides the tail: an ASA of 30 seconds can include contacts waiting five minutes. Service level with a threshold, or a percentile of wait time, is a better control target. Track ASA alongside it rather than instead of it.
- Why is my real service level lower than the calculator predicts?
- Erlang C assumes every agent is available for the whole interval, no abandons, and a steady arrival rate. Real intervals have late log-ins, unplanned breaks and peaks inside the interval. If the gap is persistent, measure your actual availability and treat the shortfall as shrinkage inside the interval, or plan at 15-minute rather than hourly resolution.
- What happens when agents are fewer than erlangs?
- Work arrives faster than it can be handled, so the queue never empties. In the model service level is zero and the average wait is infinite; in practice customers abandon, which is why the real figure is not quite zero. The Erlang A calculator models that.
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