Skip to content
ENExplore Calculators

Trunk Sizing Calculator

Work out trunk sizing instantly with clear inputs, formula shown and shareable results.

About Trunk Sizing Calculator

Trunk sizing converts busy-hour call volume and duration into erlangs, then searches for the smallest channel count meeting the target blocking probability under Erlang B. Utilisation rises with group size — a large trunk group carries traffic far more efficiently than several small ones, which is the trunking-efficiency argument for centralised SIP trunks.

Formula

Trunk sizing

traffic = calls x duration / 3600; smallest N where ErlangB(traffic, N) <= target; bandwidth = N x per-call rate

Worked example

With the default values below, Trunk Sizing Calculator returns Trunks or channels required of 31. Change any input above and every figure here is recalculated in your browser.

Inputs used

Calls in the busy hourminimum 0.01
420
Average call durationminimum 1 s
180 s
Target grade of serviceaccepts 0.001–20 %
1 %
Bandwidth per callminimum 1 kbps
95 kbps

Results

Trunks or channels required
31
Offered traffic
21.00 erlangs
Bandwidth for the trunk group
2.95 Mbps
Trunk utilisation at the target
67.7%

Step by step

  1. 1Values used
  2. 2Trunk sizing
  3. 3Trunks or channels required= 31
  4. 4Offered traffic= 21.00 erlangs
  5. 5Bandwidth for the trunk group= 2.95 Mbps
  6. 6Trunk utilisation at the target= 67.7

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges 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

What grade of service is normal?

One percent blocking in the busy hour is the usual commercial target; 0.1 percent is used for emergency and critical services.

Why is a single large trunk group better?

Trunking efficiency. Splitting 20 erlangs across two groups of ten needs more total channels than one group carrying all of it, because each group must carry its own peak.

Related Calculators

Trunk Sizing Calculator | Calcrivo