A good rule of thumb to remember is the rule of 2 and 4. On the flop (waiting for the turn), you can multiply your “outs” (card that will improve your hand to make a hand that will beat your opponent) by 4. On the turn (waiting for the river), you can multiply your “outs” by 2. The results of these equations will give you an approximately percentage for hitting that card. For example, if you need one more card to make a flush, then you have nine “outs” (the remaining nine cards of that suit). Thus, on the flop, you have approximately a 36% chance of hitting. If the turn comes and it doesn’t complete your flush, then you now have a 18% of hitting… give or take a few percentage points.

You can then use these numbers to figure out if it is “worth it” to call a bet from your opponent. Let’s say he bets $10 into a $40 pot (thus making it $50) on the flop and you need one more card to make a “good” flush. Thus, you are getting 5-to-1 on your money if you call the $10. You have (about) a 1 -in-3 chance of getting that flush (36% as computed above). Thus, the “pot odds” are there for you to call. If your opponent bet $100 (thus making the pot $140), it is not “worth it” for you to call, because you’d have to put in $100 to win $140 when you only have a 36% chance of winning. Does that make sense?