If you prefer equations to memorization, if you have x outs your pot odds are...
Definition of pot odds: The ratio of the current size of the pot to the bet that you must call. For example, if
you must call a $4 bet, and the pot has $20 in it (this includes the bettors money), then your pot odds are 20-to-4
or 5-to-1 and you should call if you have 8 outs or more (well, things get a little more complex).