Calculate busy hour call traffic in Erlangs from call volume and duration.
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.
Busy hour traffic
BHT_erlangs = calls_per_hour × avg_duration_min / 60
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.
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.
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.