Birthday Paradox Calculator
Find the chance that two people in a group share a birthday.
A famously surprising result
The birthday paradox is one of probability’s best counterintuitive results: it takes only 23 people for a coin-flip chance that two share a birthday, and 70 people for near certainty.
The trick is to compute the chance everyone is different and subtract from one; with each added person the shared-birthday probability climbs fast. Enter a group size to see it.
Fun and private
The calculation runs in your browser, so nothing you enter is uploaded.
Frequently Asked Questions
What is the birthday paradox?
The surprising fact that in a group of just 23 people, there is about a 50% chance two share a birthday.
How is it calculated?
By computing the probability that everyone has a different birthday, then subtracting from one.
Does it assume leap years?
No. It uses 365 days and assumes birthdays are evenly distributed.
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