Shared birthday calculator
What are the chances two people share a birthday?
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Why Shared Birthdays Become Likely So Quickly
The comparison is not one person against everyone else. Every pair of people in the group creates another chance for a match. As the group gets larger, the number of possible pairs grows quickly.
This calculator uses the site's historical U.S. birthday distribution, so common dates contribute slightly more probability than rare dates. It treats each person as an independent draw from that historical distribution.
How This Differs From Birthday Rarity
Birthday rarity asks how unusual one specific calendar date is. Birthday probability asks how likely it is that any two people in a group share a date. They are related birthday questions, but they measure different things.