You have a 50 per cent chance of sharing a birthday with a friend. In any group of 23 people, two people will share a birthday, according to the maths. To find the probability of everyone in the group having a unique birthday, multiply all 23 probabilities together, giving 0.493. So the probability of a shared birthday is 1 - 0.493 = 0.507, or 50.7 per cent.