tools · erlang c
Erlang Calculator for Call Center Staffing
Enter the calls you expect in an interval, how long each one takes, and the service level you have promised. The calculator returns how many agents must be on the phones, and how many to schedule once shrinkage is counted.
It runs in your browser; nothing you type is sent anywhere.
Agents needed on the phones
14
- Agents to schedule
- 20
- Expected service level
- 88.8%
- Probability a call waits
- 17.4%
- Average speed of answer
- 7.8 s
- Occupancy
- 71.4%
- Traffic intensity
- 10 erlangs
how it works
How the calculator works
Erlang C is a queueing formula published by A. K. Erlang in 1917. It assumes calls arrive at random, every caller waits until answered, and each call occupies one agent. From the workload (calls × handle time ÷ interval length, measured in erlangs) it gives the probability that a call has to wait.
The calculator adds agents one at a time until the service level meets your target and occupancy stays under your cap.
Example: 100 calls in 30 minutes at 180 seconds each is 10 erlangs. Ten agents would never catch up. Fourteen answer 88.8% of calls within 20 seconds.
inputs
What each input means
- Calls in the interval
- Calls offered, not calls answered.
- Interval length
- The window those calls arrive in. 30 minutes is the usual planning unit.
- Average handle time
- Talk, hold and after-call work, in seconds.
- Service level target and answered within
- “80% in 20 seconds” is two numbers; enter both.
- Shrinkage
- Paid time agents are not available to take calls, such as breaks, training, meetings and absence.
- Maximum occupancy
- The share of logged-in time agents spend handling calls. Capping it leaves room to breathe between calls.
limits
Where Erlang C is off
- It assumes nobody hangs up. When callers do abandon, the real queue is shorter than the formula expects, so it asks for more agents than you need.
- It assumes one agent, one contact. It does not fit chat, where an agent handles several conversations at once.
- It answers for one interval. A day’s schedule means running it interval by interval, because 9am and 2pm are different problems.
questions
Questions about Erlang C
What does Erlang C calculate?
The probability that a call has to wait, given the workload and the number of agents. Service level and average speed of answer are derived from it.
How do I calculate how many agents I need?
Work out the load in erlangs (calls × handle time ÷ interval seconds), then add agents until the service level target is met. Divide by one minus shrinkage to get the number to schedule.
What is the 80/20 rule in call centers?
80% of calls answered within 20 seconds. It is a common convention, not a standard; set the target your customers and budget support.
How many FTEs does it take to cover one seat 24/7?
168 hours a week at 40 hours per agent is 4.2 FTE. At 30% shrinkage it is 6.