How many people do you need to gather if you want at least to of them to have birthdays in the same month?

Answer 1

13, if you want to be sure. It's impossible for a group of 13 people not to have at least one pair that share a birth month, because there are only 12 months.

If you'll settle for a lower probability, thechance that a group of 5 randomly chosen people will contain at least one pair born in the same month is about 3/5, and if you gather 6 people the chance that at least two of them will share a birth month is about 4/5. Those aren't exact probabilities both because the math doesn't work out that way and and because birthdays are not randomly distributed by month ... significantly fewer people are born in February than in August. An exact probability would need to take that into account, and it's frankly more research and math than I want to do unless I'm getting paid for it.