What determines the amount of a min raise?

Like based on the rules of poker, whats the smallest amount you can raise?

I know its less then double. Always wondered about that....

the amount of the big blind, or if there a raise in front the amount of the initial raise. so for instance in a $1/$2 a raise to $4 is a min raise as you are only raising $2. If there is a raise of 10 then you have to raise atleast 10 more. I hope that makes sense

Basically a minraise is raising exactly as much as the person before you raised (where a bet counts as a raise from 0). Like steveg said.

Say person A bets 4$. You can minraise to 8$ (because he 'raised' the bet from 0 to 4 = 4$ and you raise it from 4 to 8 = 4$). If A then raises to 20$, you can minraise to 32$ (because he went from 8 to 20 = 12$ and you went from 20 to 32 = 12$).