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How to Calculate SIP Trunk Capacity: A Sizing Guide

SIP trunk capacity is measured in concurrent call channels, not total users—size it for your busiest hour with headroom, and you avoid both dropped calls and wasted spend.

Definition: A SIP channel is one concurrent call path on a trunk; trunk capacity = number of simultaneous calls you can make/receive at once.

The sizing formula

  1. Count busy-hour call attempts (BHCA) and average call duration.
  2. Estimate concurrent calls ≈ (BHCA × avg duration) ÷ 3600, then apply an Erlang/B-grade model.
  3. Add growth headroom (e.g., +20–30%).
  4. Include remote/softphone users who also consume channels for external calls.

Quick reference

Users Typical concurrent (light) With growth
10 2–3 4
50 6–8 10
200 20–25 30
500 45–55 65

Common mistakes

  • Sizing by total users, not concurrency (over-provisions).
  • Ignoring seasonal peaks (under-provisions at crunch time).
  • Forgetting inbound + outbound share the same channels.

Newrock perspective

Newrock OM-series IP-PBX shows live channel usage; pair with UMS to monitor multi-site trunks and right-size before peaks hit.

FAQ

Do inbound and outbound share channels?

Yes—total concurrent calls (both directions) can't exceed trunk channels.

What if I underestimate?

Calls busy out; most providers let you add channels quickly—monitor and adjust.

Does video use more channels?

Video is separate media but still one call; bandwidth, not channels, is the constraint.

Should I burst?

Some providers offer burst capacity; confirm terms to avoid surprise overage.

Conclusion

Size SIP trunks on busy-hour concurrency with headroom, not headcount. Monitor actual usage and adjust—capacity planning is ongoing, not one-time.

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