Erlang B Trunking Calculator
Give a group of telephone trunks the traffic it has to carry and see what share of calls is blocked, or how many trunks a grade of service needs.
Trunk counts around the target
| Trunks | Blocking | Carried (erlangs) | Target met |
|---|
One erlang is one circuit busy for the whole hour, so 120 calls of three minutes each in the busy hour come to 6 erlangs. Erlang B assumes calls arrive at random, are refused when every trunk is busy and are not retried, and gives the probability that a call is blocked: B(0) = 1 and B(n) = A × B(n−1) / (n + A × B(n−1)) for A erlangs offered to n trunks. Ten erlangs on fifteen trunks blocks 3.65% of calls and carries 9.64 erlangs; a 1% grade of service needs eighteen trunks, because sixteen and seventeen still block 2.23% and 1.29%. Carried traffic is A × (1 − B) and the blocked share is A × B. Everything runs locally in your browser with no storage, uploads or network access.