Alex Waese-Perlman

The birthday paradox is the unexpectedly high probability of two people sharing a birthday in a group.

This calculator allows large numbers of people and days. It's useful for determining the probability of a hash collision. Use the check on the left to select the independent variable.

Number of people:

Number of days in a year:

Probability of a collision:

It's able to handle large numbers (over 50 digits) by working with logarithms and using the Stirling approximation