# SIP channel calculator

Work out how many SIP channels a trunk needs from the calls in your busiest hour, using the Erlang B formula that telephone networks have been sized with for a century, and the bandwidth those calls take.

SIP channels needed

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## How the calculation works

A SIP channel carries one call at a time, so a trunk needs enough channels for the calls happening at once in its busiest hour. Calls do not arrive evenly, so a trunk sized for the average is busy far more often than it looks.

- **Traffic in erlangs.** One erlang is one channel busy for a whole hour. Calls per hour multiplied by the average call length in hours gives the traffic: 60 calls of 3 minutes is 3 erlangs.
- **Erlang B.** For a given traffic and number of channels, the Erlang B formula gives the share of calls that find every channel busy and are blocked. The calculator finds the fewest channels that keep blocking at or below your target.
- **Bandwidth.** Each call needs bandwidth in both directions at the same time. The figures include IP, UDP and RTP overhead at 20 ms packets; Ethernet framing and VPN or tunnel headers add a little more.

Measure your busiest hour from your phone system's call records rather than estimating it, and remember that a held, transferred or conferenced call can use more than one channel. Erlang B assumes blocked calls go away; queues and call-backs need Erlang C.

## SIP trunks and hosted voice

We provide SIP trunks and hosted voice to partners, with channels sized to your traffic and burst capacity when you need it.

[Our capabilities](https://netavo.com/services)

[Work with us](https://netavo.com/work-with-us)

Source: https://netavo.com/tools/sip-channel-calculator