There are two ways to do this, my preferred method is called the four-two method. The 4:2 says that after the flop you multiply your outs by 4 to get your chance to catch what you need and by two after the turn. In other words, lets say you are dealt an Ad, 2d (ace and two of diamonds) and two diamonds show up on the flop. There are nine outs that can help you so your odds of catching the flush at this point are roughly 9x4 or 36%, refer to the following chart to get the exact number but, at the table, the difference between a percentage point or two are really meaningless. If your chances are roughly 30%, there's $500 in the pot and the bet is $20 then pot odds tell you to at least toss in the double sawbuck and roll the dice. Whether you call, bump a little, make a big bet or go all in is beyond the scope of this article!
Or, if you want to go and get all technical, try and memorize this table before your next game:
| Outs | Post Flop% | Turn% |
| 2 | 8% | 4% |
| 3 | 13% | 7% |
| 4 | 17% | 9% |
| 5 | 20% | 11% |
| 6 | 24% | 13% |
| 7 | 28% | 15% |
| 8 | 32% | 17% |
| 9 | 35% | 19% |
| 10 | 38% | 22% |
| 11 | 42% | 24% |
| 12 | 45% | 26% |
| 13 | 48% | 28% |
| 14 | 51% | 30% |
| 15 | 54% | 33% |
| 16 | 57% | 34% |
| 17 | 60% | 37% |
| 18 | 39% | 62% |
| 19 | 41% | 65% |