How to Model Waiting Times with EXPON.DIST in Excel

Use EXPON.DIST to estimate how long until the next customer, failure or event when arrivals are random.

You will learn to

• Find the chance the next arrival comes within a time
• Find the chance of a long gap
• Find the chance a wait falls between two times

How it works

EXPON.DIST models the time between random events — the gap between customers, between machine failures, between support tickets. Its one parameter, lambda, is the rate: events per unit of time, which is 1 divided by the average gap.

With cumulative TRUE it returns the chance the wait is x or shorter. For a long wait use 1 minus that, and for a wait between two times subtract two cumulative results. Always keep the time units consistent: if the average gap is in minutes, x must be in minutes too.

=EXPON.DIST(2,1/4,TRUE)