SIP Trunk Capacity Calculator
Size a SIP trunk from busy-hour call attempts and hold time: Erlangs, concurrent channels and the bandwidth they consume.
Inputs
Sets the safety factor added on top of the offered load.
Concurrent Channels Required
113
Offered Traffic
90.00Erlangs
Trunk Bandwidth
10.35Mbps
Channel Utilisation at Design Load
79.6%
Step by step
Values used
Busy-hour call attempts = 1,800 calls/hour; Average hold time = 180 seconds; Codec (bandwidth per call on Ethernet) = G.711 — 87.2 kbps; Grade of service (blocking target) = P.01 — 1% blocking; Signalling and RTCP allowance = 5 %
SIP Trunk Capacity
Erlangs A = BHCA × AHT ÷ 3600; channels N ≈ A + z√A; trunk bandwidth = N × codec rate × (1 + signalling allowance).
Per-call bandwidth
G.711 on Ethernet = (160 B payload + 40 B RTP/UDP/IP + 18 B Ethernet) × 8 × 50 pps = 87.2 kbps.
Concurrent Channels Required
= 113
Offered Traffic
= 90.00 Erlangs
Trunk Bandwidth
= 10.35 Mbps
Channel Utilisation at Design Load
= 79.6
How it works
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.
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.