The Excel POISSON function returns the Poisson probability of a number of events in a fixed interval. It is a legacy name — it still works, but Microsoft now recommends POISSON.DIST, which is identical in behaviour.
Syntax
| Argument | Description | |
|---|---|---|
x | Required | The number of events (a non-negative integer). |
mean | Required | The expected number of events (λ), a positive number. |
cumulative | Required | TRUE returns the cumulative probability (0 through x); FALSE returns the probability of exactly x. |
How to use it
POISSON models how often a rare event happens in a fixed window when events are independent. Pass the count x, the average rate mean, and a cumulative flag.
With cumulative FALSE you get the probability mass at exactly x; with TRUE you get the running total from 0 up to and including x.
Use the modern name: =POISSON.DIST(x, mean, cumulative) takes the same arguments and returns the same numbers. POISSON is kept only so old workbooks keep calculating.
Try it: interactive demo
Pick a POISSON example to see the formula and its result.
Practice workbook
Frequently asked questions
Is POISSON the same as POISSON.DIST?
POISSON.DIST (introduced in Excel 2010) takes the same x, mean, and cumulative arguments and returns identical results. POISSON is retained only for backward compatibility.What does the cumulative argument do?
Can x or mean be a decimal?
mean can be any positive number. x is truncated to an integer, and it must be non-negative or the function returns an error.Should I switch to POISSON.DIST?
.DIST family. Existing POISSON formulas will keep working.Master functions like this in one day
This page covers one function. Our Excel Formulas and Functions class covers the 30 that matter most — live, hands-on, taught by professionals in Dallas–Fort Worth, Houston, Austin, Oklahoma City, Denver, or online.
See the Formulas & Functions Class