User blog comment:NOBODY/A new Pixel Love game on Friday?!/@comment-4637670-20120824184421/@comment-1455034-20120825132220

Try reading the Bulbapedia article on Catch Rate: Catch Rate. The best formula on the page is the modified catch rate, which should be simple to understand.

There is also a formula for the probability of catching a Pokemon (when you know its catch rate), but due to the long formula (and how knowing the probability won't really help), I would suggest you skip that (as you likely won't know how the various mathematical things work). The Poke Ball effectiveness table also comes in helpful.