Erlang B calculator for SIP trunk channels
Enter your busy-hour calls and average call length to see how many simultaneous channels you need before callers start getting a busy signal.
All 13 channels busy with G.711 calls need about 1.13 Mbps each way (87.2 kbps per call, Ethernet included). Work out other codecs.
| Channels | Calls blocked | Meets target |
|---|---|---|
| 11 | 2.30% | No |
| 12 | 1.14% | No |
| 13 ← needed | 0.52% | Yes |
| 14 | 0.22% | Yes |
| 15 | 0.09% | Yes |
How this calculator works
Erlang B is the standard formula for sizing phone lines and SIP trunk channels. It assumes that a caller who finds every channel busy gets a busy signal and goes away (is “blocked”), rather than waiting in a queue. You give it the traffic in your busiest hour and the share of blocked calls you can live with, and it finds the fewest channels that meet that target.
Working out your traffic
Traffic in erlangs is the calls in the busy hour multiplied by the average call length in hours. 120 calls of 3 minutes each is 120 × 3 ÷ 60 = 6 erlangs: on average, six lines are in use at once. At 1% blocking, 6 erlangs need 13 channels. That is more than the average because calls arrive in clumps.
Channels, trunks and bandwidth
A SIP channel is one simultaneous call. Providers usually price per channel, so over-sizing costs money and under-sizing costs calls. Once you know the channel count, check the bandwidth with the codec bandwidth calculator: G.711 needs roughly 87 kbps per call in each direction.
Limits
Erlang B assumes blocked callers do not try again. In practice many redial straight away, so it can slightly underestimate what you need. Use your busiest realistic hour, not the daily average. If callers wait in a queue for an agent, use Erlang C instead.
FAQ
What is the difference between Erlang B and Erlang C?
Erlang B assumes callers who find no free line give up and are blocked, which suits phone lines and SIP trunks. Erlang C assumes callers wait in a queue for an agent, which suits call center staffing.
What blocking rate should I use?
1% is the most common design target for business lines: 1 call in 100 gets a busy signal in the busy hour. Use 0.1% to 0.5% where a missed call is costly, and 2% to 5% for lines that matter less.
Do I need one channel per user?
No. Not everyone is on a call at the same time. Size for the number of simultaneous calls in your busy hour, not for the number of staff.