Work out how many phone lines, SIP trunk channels or IVR ports you need for your busiest hour, and how many calls get a busy signal at the count you have now.
Sizing agents rather than lines? Use the Erlang C calculator. Connecting your own trunk to a voice agent? See the SIP trunk docs.
Inbound and outbound calls that need a line at the same time.
Decimals are fine: 2.5 is two and a half minutes.
Leave at 0 to skip.
Lines needed
18
for 1% blocking or less
Busy-hour traffic
10.00 erlangs
Blocking at that count
0.71%
Share of calls that find every line busy
Blocked calls per busy hour
1.43
Lines needed at each blocking target
| Target blocking | Lines needed | Actual blocking |
|---|---|---|
| 0.1% | 21 | 0.09% |
| 0.5% | 19 | 0.37% |
| 1% | 18 | 0.71% |
| 2% | 17 | 1.29% |
| 5% | 15 | 3.65% |
Assumptions
If callers wait in a queue for an agent instead of hearing a busy tone, that is Erlang C. Use the Erlang C calculator to size agents, then this one to size the lines they need.
Both start from the same busy-hour traffic in erlangs. They answer different questions.
A contact center usually needs both: Erlang C for the agents, then Erlang B for the lines, with extra lines for the callers waiting in the queue.
Traffic in erlangs is calls in the busy hour times average minutes, divided by 60. 200 calls of 3 minutes is 10 erlangs. At 1% blocking, 10 erlangs needs 18 lines, not 10, because calls do not arrive evenly. With 15 lines, about 3.65% of calls would be blocked.
Use your busiest hour, not a daily average. If you only have daily calls, look at an hourly report for the peak, or the talk time calculator for minutes.
More on trunks: SIP trunk providers in India for AI calling.
Enter your busy-hour calls and average call length. The calculator finds the fewest SIP trunk channels that keep blocking at or below your target. For example, 200 calls of 3 minutes (10 erlangs) needs 18 channels for 1% blocking. Each channel carries one call at a time.
One erlang is one line in use for a full hour. 60 one-minute calls in an hour, or 20 three-minute calls, is 1 erlang. Traffic in erlangs is busy-hour calls times average minutes, divided by 60.
1% is a common choice for business lines and IVRs. Pick 0.1% or 0.5% when a lost call is expensive, such as sales or emergency lines, and 2% to 5% for low-priority outbound lines. Lower blocking needs more lines; the table shows the count for each target.
Erlang B assumes a call that finds every line busy is lost, so it sizes lines and trunks. Erlang C assumes the caller waits in a queue, so it sizes agents. Use the Erlang C calculator for agents and this one for the lines they use.
No. Erlang B assumes blocked calls are lost and not retried. If many blocked callers redial at once, real traffic is higher than you entered, so add some margin or pick a lower blocking target.
Yes. The line count is the number of calls that must be handled at the same time, which is the concurrency you plan for on an IVR or a voice agent, and the channel count on the SIP trunk behind it.
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Lines needed for 0.1% to 5% blocking
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Blocked calls per busy hour
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