Birthday Paradox: Why Just 23 People Can Share a Birthday

What Is the Birthday Paradox?

The birthday paradox is a probability puzzle showing that in a group of just 23 people, there is about a 50% chance that two people share the same birthday.

Why Does the Birthday Paradox Happen?

The probability rises quickly because each new person can match with everyone already in the room, creating many possible birthday pairs.

Most people know the birthdays of their close family members or friends.

But when you’re in a crowd of people, something surprising happens.

Now that I think about it, I actually know someone with the same birthday as me, and the same age. A friend's brother I’ve known since I was 22.

And speaking of probability, here’s another funny coincidence: this year my birthday falls on a Thursday, and guess what day I was born on… a Thursday.

Thanks to the Birthday Paradox, in a group of just 23 people, there’s about a 50.7% chance that at least two people will share the same birthday.

This happens because the number of possible birthday comparisons grows quickly as more people are added to a group. Even though there are 365 days in a year, the number of ways people can match each other increases much faster than we expect.

By the time a group reaches:

• 30 people → about a 70% chance of a shared birthday
• 40 people → about a 90% chance

Probability can be strange sometimes. Do you remember the first time you noticed two people sharing the same birthday?

And speaking of birthdays…

At Danna Bananas, we carry funny gifts that ship across Canada, including funny socks, novelty mugs, and quirky surprises.

If you’re celebrating someone special, funny birthday gifts are always a hit.

If you’re celebrating someone special, funny birthday gifts are always a hit.

Looking for funny Mother’s Day gift ideas in Canada? Check out our latest gift guide.