The decision to call a raise will most always be based upon pot odds. For instance, if the pot contains $30 and you need to make a call of $10 to stay in the hand, the pot odds are $30 to $10 or 3:1. To convert this to a percentage, you add the size of the pot part of the ratio (3) to the call part of the ratio (1) to get 4. To get the percentage you divide the sum 4 by 1 which is 25% (1/4).

Here is a handy chart that lists the number of outs you have (cards that will make your hand) and the percentage chance of making your hand.