Birthday Paradox Calculator

The Birthday Paradox is a famous probability result: in a group of just 23 people, there is a greater than 50% probability that two people share a birthday. This surprises most people because 23 seems small compared to 365 days

The calculator uses the standard published formula for birthday paradox. Results are accurate to the precision of the inputs you provide. For financial, medical, or legal decisions, always verify with a qualified professional.

This calculator works with inches, percentages. You can enter values in the units shown — the calculator handles all conversions internally.

The core formula is: P(at least one shared birthday) = 1 − P(all different). Each step in the calculation is shown so you can verify the result manually.