One room on the calendar ring23 PEOPLE
←→ step through the room · Esc clear
Chance of a shared birthday, 2 to 100 peoplen = 23 → 50.73%
theory
measured runs
click or arrow-key the chart to move the group size
1000 rooms, one square eachn = 23
shared birthdayall different
Rooms with a match—
Measured—
Theory50.73%
Gap—
Run a thousand rooms to test the number on the left.
The odds right now
50.73%
chance that at least two of the 23 people share a birthday
Pairs in the room253
Matches your day5.86%
Controls
23
Why 23 is the surprise
Ask around and most people guess about 180 — half of 365. That answers a different question: how many strangers you need before one matches your birthday. In this room that chance is only 5.86%.
Nobody is the reference point. Every pair counts, and 23 people quietly make 253 pairs — each one its own little chance to collide. That is why the answer is 23 and not 180.