The app referenced is a nice encapsulation of the formula in a form that is immediately usable without significant understanding.
If you find this interesting and want to know how these things are calculated, you can find the full calculations in a blog post I wrote a few years ago:
If you find this interesting and want to know how these things are calculated, you can find the full calculations in a blog post I wrote a few years ago:
https://www.solipsys.co.uk/new/TheBirthdayParadox.html?si21h...
There are some previous discussions:
https://news.ycombinator.com/item?id=19296265
https://news.ycombinator.com/item?id=4753014