A unit of measurement of traffic density in a telecommunications system. The erlang describes the total traffic volume of one hour, or 3600 seconds.

For example, 60 calls in one hour, each lasting 5 minutes, results in the following number of erlangs:
minutes of traffic in the hour = number of calls x duration
minutes of traffic in the hour = 60 x 5
minutes if traffic in the hour = 300
hours of traffic in the hour = 300/60
hours of traffic in the hour = 5
traffic figure = 5 erlangs

Network designers use the erlang to understand traffic patterns within a voice network and use the figures to determine how many lines are required between a telephone system and a central office or between network locations.

Erlang calculations are further broken down as follows:

Erlang B — the most commonly used traffic model. Erlang B is used to work out how many lines are required if the traffic figure during the busiest hour is known. This model assumes that all blocked calls are cleared immediately.
Extended Erlang B — similar to Erlang B, this model can be used to factor in the number of calls that are blocked and immediately tried again.
Erlang C — this model assumes that all blocked calls are queued in the system until they can be handled. Call centers can use this calculation to determine how many call agents to staff, based on the number of calls per hour, the average duration of class and the amount of time calls are left in the queue.

The erlang is named after the Danish telephone engineer A. K. Erlang.

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