Erlang B trunk calculator.

Worked example

300 calls an hour at 120 seconds each is 300 x 120 / 3,600 = 10 erlangs. For no more than 1% blocking you need 18 trunks (blocking 0.714%), and each trunk is busy about 55.2% of the time. One trunk fewer would block more than 1%.

How it works

The tool uses the standard recurrence from the Erlang B formula: B(E, 0) = 1 and B(E, j) = E x B(E, j-1) / (j + E x B(E, j-1)). It steps j up until B is at or under your target. Carried traffic is E x (1 - B) and utilisation is carried traffic divided by trunks.

Assumptions. Poisson arrivals from a very large population, no queue (a blocked call is lost, not retried) and no lines held for other uses. If blocked callers redial, real traffic is higher than the offered figure you enter.

FAQ

What is the difference between Erlang B and Erlang C?

Erlang B assumes blocked calls are lost and gives the chance a call is blocked. Erlang C assumes blocked calls wait in a queue and gives the chance a call has to wait.

What is an erlang?

One erlang is one circuit or agent kept busy for a full hour. Offered traffic in erlangs is the arrival rate times the average holding time, in the same time units.

What blocking target should I use?

It depends on what a lost call costs you. The tool accepts any target between 0% and 100% and shows the trunks needed and the blocking you get.

Does Erlang B work for SIP trunks and VoIP channels?

It can size a count of simultaneous channels if calls arrive randomly and blocked calls are cleared. It does not model bandwidth limits on the link.