Offered traffic: erlangs. Bars show how the chance of blocking or waiting falls as capacity rises.
How the Erlang calculator works
An erlang is one hour of continuous traffic. Offered traffic equals calls per hour times average call length in seconds, divided by 3,600. This Erlang calculator then applies two standard models.
Erlang B: how many phone lines do you need?
Erlang B assumes a call that finds every line busy is lost. Enter your peak calls per hour and average call length, choose how much blocking you accept (1 percent is common), and the calculator returns the number of lines or SIP trunk channels. Read what SIP trunking is before you buy channels.
Erlang C: how many agents do you need?
Erlang C assumes callers wait in a queue. It estimates the probability that a call waits, then the service level (for example 80 percent answered within 20 seconds), average speed of answer and agent occupancy. Shrinkage converts the agents on phones into the number you need to schedule.
| Model | Use it for | Key input |
|---|---|---|
| Erlang B | Phone lines, SIP trunk channels | Blocking probability |
| Erlang C | Call center agents | Service level and answer time |
Next, check capacity for your network with the VoIP bandwidth calculator, or learn about PBX systems.
Erlang calculator FAQ
What is an erlang?
One erlang is one hour of continuous traffic on one line or agent. You calculate it as calls per hour multiplied by average call length in seconds, divided by 3,600.
What is the difference between Erlang B and Erlang C?
Erlang B assumes blocked calls are lost, so it sizes phone lines or trunks. Erlang C assumes callers wait in a queue, so it sizes call center agents.
What service level should I target?
A common target is 80 percent of calls answered within 20 seconds, but the right target depends on your business and customers.
What is shrinkage?
Shrinkage is the share of paid time agents are not available for calls, for example breaks, training and absences. The calculator adds it to estimate how many agents to schedule.
Does this replace workforce management software?
No. It gives a quick estimate for a single interval. Real staffing also needs forecasts, schedules and varying call patterns.
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