The Board:

Calculating Odds
In this exampel, you need one more heart to make your hand. There are 9 outs(cards that improve your hand) left in the deck and there are 47 cards not showing. Your Odds of making a flush on the next card is 9/(47-9), or 1 out of 4, approximately.
If there is $20.00 in the pot, and it will cost you $4.00 to call, you are getting the correct odds to call. On the other hand, if there is only $12.00 in the pot, you are not getting correct pot odds to call.

For ease of calculating Pot Odds, we have compiled this list of Odds Against Making your Draw: