VoIP Erlang B Lines Calculator

Erlang B answers one question: how many lines do you need so that only a small share of calls in the busiest hour get a busy signal?

• Traffic: calls in the busy hour x average call duration in seconds / 3600. One Erlang is one line kept busy for a full hour.
• Lines: the calculator tries 0, 1, 2 and more lines and stops at the first count where the Erlang B blocking probability is at or below your target.
• Calculated blocking is the blocking at that line count. It is usually a little below the target, because lines come in whole numbers.

The default is 350 calls in the busy hour, 180 seconds per call and a 1% blocking target:

• Traffic: 350 x 180 / 3600 = 17.5 Erlangs
• 26 lines would block 1.33% of calls, which is above the target
• 27 lines block 0.85%, so the answer is 27 lines

A stricter 0.1% target needs 32 lines. A looser 5% target needs 23.

Use the call records from your phone system and pick the busiest hour of a normal busy day. An average over the whole day will undersize the trunk.

1% (often written P.01) is a common target for business lines. Use a lower target where a busy signal is costly, such as sales or support lines.

No. Erlang B assumes a blocked call is dropped, not held. Call centers where callers wait on hold are sized with Erlang C, which this calculator does not do.

Yes. For VoIP, a line is one call at a time: one SIP trunk channel or one concurrent call. Bandwidth per call depends on the codec and is not calculated here.