23 people give 50% for dates, but what about months or weekdays?
This tool takes the birthday problem off the fixed 365-day cycle and computes it for any cycle length N. The formula applied is P(match) = 1 - (N/N) x ((N-1)/N) x ... x ((N-m+1)/N), with presets for 12 months, 7 weekdays, 31 days of the month and 24 hours, plus a custom value. It also solves back for the group size a target probability needs, raising the count one person at a time until the target is first passed. Check values: 23 people over 365 slots give 50.7297%, 4 people over 12 slots give 42.7083% and 4 over 7 slots give 65.0146%.
The model assumes every slot is equally likely and leaves out leap day. When there are more people than slots the pigeonhole principle forces a match, the probability is 100%, and the result says so. Products are summed in log space, so large groups do not collapse to zero.
The chance that three or more people share one slot, and the chance that someone matches one specific date you name, are not covered. Month-by-month variation in real births is not modelled either. Reference and study use as of October 2026; unlike a draw tool, this page uses no random numbers at all.
Frequently Asked Questions
Yes. Pick 12 slots for months, 7 for weekdays, 31 for days of the month, 24 for hours, or type any number. Only N changes in the same formula. Among 4 people the chance of a shared month is 42.7083% and of a shared weekday 65.0146%.
No closed-form shortcut is used. The group size rises one person at a time until the probability first passes the target. For 365 slots that is 23 people for 50% and 57 people for 99%.
The calculation assumes every slot is equally likely and leaves out leap day. When real births cluster in certain periods, the true chance of a match moves slightly above the calculated value.