SIP Trunk Capacity Calculator
Size a SIP trunk from busy-hour call attempts and hold time: Erlangs, concurrent channels and the bandwidth they consume.
About SIP Trunk Capacity Calculator
Offered traffic in Erlangs is simply the call-seconds carried in the busy hour divided by 3600. The channel count uses the normal approximation to Erlang B — the offered load plus a safety factor scaled to the square root of the load — then multiplies by the on-the-wire rate of the codec you selected. Under-provisioned SIP trunks return 503 responses at the worst possible moment, and over-provisioned ones are billed per channel every month.
Formulas
SIP Trunk Capacity
Erlangs A = BHCA × AHT ÷ 3600; channels N ≈ A + z√A; trunk bandwidth = N × codec rate × (1 + signalling allowance).
- A
- Offered traffic in Erlangs
- z
- Blocking-target safety factor
- N
- Simultaneous call channels
Per-call bandwidth
G.711 on Ethernet = (160 B payload + 40 B RTP/UDP/IP + 18 B Ethernet) × 8 × 50 pps = 87.2 kbps.
Worked example
With the default values below, SIP Trunk Capacity Calculator returns Concurrent Channels Required of 113. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Busy-hour call attemptsaccepts 1–500,000 calls/hour
- 1,800 calls/hour
- Average hold timeaccepts 1–7,200 seconds
- 180 seconds
- Codec (bandwidth per call on Ethernet)one of G.711 — 87.2 kbps, G.722 — 87.2 kbps, G.726-32 — 55.2 kbps, G.729 — 31.2 kbps, Opus 24k — 47.2 kbps
- G.711 — 87.2 kbps
- Grade of service (blocking target)one of P.01 — 1% blocking, P.005 — 0.5% blocking, P.001 — 0.1% blocking
- P.01 — 1% blocking
- Signalling and RTCP allowanceaccepts 0–50 %
- 5 %
Results
- Concurrent Channels Required
- 113
- Offered Traffic
- 90.00 Erlangs
- Trunk Bandwidth
- 10.35 Mbps
- Channel Utilisation at Design Load
- 79.6%
Step by step
- 1Values used
- 2SIP Trunk Capacity
- 3Per-call bandwidth
- 4Concurrent Channels Required= 113
- 5Offered Traffic= 90.00 Erlangs
- 6Trunk Bandwidth= 10.35 Mbps
- 7Channel Utilisation at Design Load= 79.6
Assumptions and limits
- The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is SIP Trunk Capacity calculated?
Erlangs A = BHCA × AHT ÷ 3600; channels N ≈ A + z√A; trunk bandwidth = N × codec rate × (1 + signalling allowance). Offered traffic in Erlangs is simply the call-seconds carried in the busy hour divided by 3600. The channel count uses the normal approximation to Erlang B — the offered load plus a safety factor scaled to the square root of the load — then multiplies by the on-the-wire rate of the codec you selected.
Why does SIP Trunk Capacity matter?
Under-provisioned SIP trunks return 503 responses at the worst possible moment, and over-provisioned ones are billed per channel every month.
What values do I need to enter?
This calculator takes 5 inputs: Busy-hour call attempts, Average hold time, Codec (bandwidth per call on Ethernet), Grade of service (blocking target), Signalling and RTCP allowance. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.