Birthday Paradox Calculator
Find the chance two people share a birthday.
Surprisingly few people are needed for a shared birthday to be likely.
The math behind it
P(shared) = 1 − probability all are different = 1 − ∏(365−i)/365.
Worked example
23 people → 50.7% chance of a match.
FAQ
Why so few?
It counts every pair, not just matches to one person — pairs grow fast.