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Chance of sharing a birthday

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Probability index
Chance of sharing a birthday diagram
The chance of two people in a group sharing the same birth date rises surprisingly fast as the size of the group increases. Of course, the chance is zero if there is only one person and 100% if there are 366 people. This calculation does not take leap years into account but these have a very small effect.
No. of people

Chance that two share a birthday
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People are surprised that it only takes a group of about 20 to get a 50/50 chance of a shared birthday. The probability is only fractionally off 100% once the group reaches 100 people. This is because they do not realise how many potential pairings there are in even a small group of people.