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Busy Hour Call Attempts Calculator

Work out busy hour call attempts instantly with clear inputs, formula shown and shareable results.

About Busy Hour Call Attempts Calculator

Busy hour call attempts is the standard capacity metric for call servers, because signalling processing scales with attempt rate rather than with talk time. Erlangs measure the media path load instead. Both matter: a system can have ample trunk capacity yet be limited by the SIP proxy's attempts-per-second ceiling.

Formula

BHCA

BHCA = subscribers x calls per day x busy-hour share; erlangs = BHCA x average duration / 3600; cps = BHCA / 3600

Worked example

With the default values below, Busy Hour Call Attempts Calculator returns Busy hour call attempts of 5,100. Change any input above and every figure here is recalculated in your browser.

Inputs used

Subscribers or extensionsminimum 1
5,000
Calls per subscriber per dayminimum 0.01
6
Share of calls in the busy houraccepts 0.1–100 %
17 %
Average call durationminimum 1 s
180 s

Results

Busy hour call attempts
5,100 BHCA
Busy hour traffic
255.00 erlangs
Call attempts per second
1.417 cps
Traffic per subscriber
0.05100 erlangs

Step by step

  1. 1Values used
  2. 2BHCA
  3. 3Busy hour call attempts= 5,100 BHCA
  4. 4Busy hour traffic= 255.00 erlangs
  5. 5Call attempts per second= 1.417 cps
  6. 6Traffic per subscriber= 0.05100 erlangs

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 5 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

Why do vendors rate systems in BHCA?

Because call setup and teardown is the expensive processing step. A short call costs almost as much signalling as a long one, so attempts drive CPU load.

What is a typical erlang figure per subscriber?

Around 0.03 to 0.10 erlangs per business extension in the busy hour, meaning each extension is on a call 3 to 10 percent of the time.

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