About this calculator
When to use Poisson
The Poisson distribution models the number of times an event happens in a fixed interval of time or space, such as calls per hour or typos per page, when events happen independently at a steady average rate μ (sometimes written λ).
For a Poisson distribution the mean and the variance are both μ, so the standard deviation is √μ. There's no upper limit on X, so "at least" problems use the complement.