The Excel POISSON.DIST function returns the Poisson probability — the chance of a given number of events occurring in a fixed interval when events happen at a known average rate.
Syntax
| Argument | Description | |
|---|---|---|
x | Required | The number of events (a non-negative integer). |
mean | Required | The expected number of events in the interval (lambda). Must be positive. |
cumulative | Required | TRUE returns the cumulative probability of x or fewer events; FALSE returns the probability of exactly x events. |
How to use it
POISSON.DIST models counts of independent events — calls per hour, defects per batch, arrivals per minute — given an average rate mean. Set cumulative to choose which question you are asking.
With FALSE you get the probability mass at exactly x events; with TRUE you get the running total for 0 through x events. The mean is both the average and the variance of a Poisson distribution.
Modern vs legacy name: POISSON.DIST (Excel 2010+) replaces the older POISSON, which is kept as a Compatibility function. Both compute the same values; new workbooks should use the dotted name.
Try it: interactive demo
Pick a POISSON.DIST example to see the formula and its result.
Practice workbook
Frequently asked questions
What does the cumulative argument do in POISSON.DIST?
FALSE returns the probability of exactly x events; TRUE returns the cumulative probability of x or fewer events (0 through x added together).When should I use a Poisson distribution?
What is the mean argument and what are its constraints?
Is POISSON.DIST the same as the old POISSON?
POISSON is retained as a Compatibility function for older workbooks; POISSON.DIST is the current name introduced in Excel 2010.Master functions like this in one day
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