[isostatic]

How confident? Even with an even distribution of birthdays it's possible to have a billion people in the room that don't share your birthday. Very unlikely, but as there's only about 20 million people with your birthday, and 7 billion without, it's quite doable.

[dwaltrip]

"Very unlikely" is a severe understatement. With only 100k people, the odds of no one else having your birthday is 10^-120. With a million people, it is 10^-1192.

And with a billion people... apparently, there are over a million zeroes between the decimal place and the first non-zero number. That is a pretty damn small probability.

Here is the code I used in python 3:

   from decimal import Decimal
   print((Decimal(364) / Decimal(365)) ** num_people)

[isostatic]

quite, hence "how confident"

There's other things as play. Imagine you, born on July 15th, walked into a room with 1000 people. You'd be fairly confident someone will be born on July 15?

What if I then told you it was the annual astrology get together of Capricorns?

[dwaltrip]

Sure, but you did say "even distribution".