Birthday Paradox Calculator
Enter the number of people and possible days to calculate the shared-birthday probability.
Assumes independent, identically distributed birthdays with each entered day equally likely. Not a real-population estimate.
A QUICK WALKTHROUGH
How to use this tool
- Enter the number of people in the group.
- Enter the number of equally likely days in the model.
- Review the probability that at least two people share a day.
Explicit assumptions
The calculation assumes independent and identically distributed birthdays, with each of the entered days equally likely. It is a mathematical model, not a real-population estimate.
Probability only
It does not account for seasonal birth patterns, leap days, locations, demographics, or real-world records.
GOOD TO KNOW
Common questions
What does the result mean?
It is the modeled probability that at least two people share one of the entered equally likely days.
Can I change the number of days?
Yes. Enter the day count for the model you want to examine.