Busy Hour Traffic
Calculate busy hour call traffic in Erlangs from call volume and duration.
About Busy Hour Traffic
One Erlang represents one channel/circuit continuously occupied for one hour — so busy hour traffic in Erlangs is simply the total call-minutes carried during the busiest hour of the day divided by 60. This is the foundational traffic engineering metric used to size the number of trunks, channels, or circuits a telecom network needs, since it captures both call volume and call duration in a single unit that's directly comparable against circuit capacity.
Formula
Busy hour traffic
BHT_erlangs = calls_per_hour × avg_duration_min / 60
Worked example
With the default values below, Busy Hour Traffic returns Busy Hour Traffic (BHT) of 30.00. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Calls per Busy Hourminimum 0
- 600
- Average Call Durationminimum 0.01 min
- 3 min
Results
- Busy Hour Traffic (BHT)
- 30.00 Erlangs
- Total Call Minutes in the Busy Hour
- 1,800.0 min
Step by step
- 1Values used
- 2Busy hour traffic
- 3Busy Hour Traffic (BHT)= 30.00 Erlangs
- 4Total Call Minutes in the Busy Hour= 1,800.0 min
Assumptions and limits
- The result is derived only from the 2 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 exactly is an Erlang?
An Erlang is a dimensionless unit of telecommunications traffic intensity representing one call channel occupied continuously for one hour. 1 Erlang could mean one call lasting exactly 60 minutes, or two calls lasting 30 minutes each, or 20 calls lasting 3 minutes each — what matters for capacity planning is total occupied-channel-minutes, not the number of individual calls.
Why does traffic engineering focus on the 'busy hour' specifically?
Network capacity must be sized to handle the peak, not the average — a trunk group sized for average daily traffic would experience heavy congestion during the busiest hour (often mid-morning or early afternoon for business traffic). The busy hour is typically defined as the single 60-minute period with the highest traffic volume across a representative day.
How is BHT used after it's calculated?
BHT in Erlangs is the direct input to the Erlang B or Erlang C formulas, which determine how many trunks/channels are needed to keep call blocking probability below a target threshold (e.g. 1-2% for typical enterprise trunking) — see the Call Blocking Calculator for that next step.