Erlang C Formula

For good forecasting it is not necessary to have in depth knowledge about the Erlang C formula. Below we explain how the formula mathematically works. Our technicians have used this knowledge in creating our forecasting and scheduling services so you do not have to worry about it.


The Erlang C formula is in its original form defined as a function of two variables: the number of agents N and the load A. On the base of these values we can calculate the probability P that an incoming call will not be served immediately and therefore the call has to wait in the waiting queue.

The Erlang C Formula treats incoming calls as

  • poisson arrivals
  • exponentially distributed service times
  • callers will not abandon calls while waiting for an agent

Study describing when the Erlang C formula does well forecasting real situations (PDF)

Erlang C Formula as described by WikiPedia

Improve your 9-1-1 service by answering the pone; Using Erlang C to compare PSAPS

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