Erlang C

This formula is considered the “standard” by which many contact centers operate. Developed by Danish mathematician A.K. Erlang in 1917 who studied delays in telephone traffic in central switching stations. It has become accepted by many industries as the first step in accurately predicting the number of servers required for many types of applications. "Server" in this case is someone or something that serves a client that arrives at  random intervals, but the probability of arrival during a given interval is known. A "server" can be an agent in a contact center, an automated tollbooth on a bridge, a teller or ATM at a bank, etc. It returns the probability of a client waiting for an available resource.