A unit used to describe traffic density in telephony, named for A. K. Erlang (1878-1929), a Danish mathematician. It has several related senses:


A unit of traffic intensity: the average number of simultaneous connections observed during a measurement period.

ANSI/IEEE Standard 312-1977.
Standard Definitions of Terms for Communication Switching.


A unit of message loading, used to describe usage of leased facilities. The number of call-seconds divided by 3,600, or in other words, if 1 circuit was in continuous use for 1 hour 1 erlang of use would have occurred.

IEEE Standard 599-1983.
Standard Glossary of Power Systems Data Transmission and Related Channel Terminology.


A collection of interactive calculators for determining erlangs:

A brief biography of Erlang: 

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