Lookie here:

http://www.foldorraise.com/outscalculator.php

I used to pretty much calculate my odds like this: 9 outs, 2% per out, 18% + 1% for the 0.1%s so 19% for each draw left(turn + river / 19% + 19% = 38%). But according to the link above, its 19% for the turn, 20% for the river (which I understand - because there is 1 less card in the deck, right?), but the chance of hitting either is 35%. The higher up you go, the less accurate the turn + river method is. For example, with 22 outs, 47% on the turn, 48% on the river, the chance of hitting either is only 72%.

After thinking about it a minute I realized why. If you flipped a coin, you would have a 50% chance of hitting or "drawing" a heads, and in two flips a 75% chance (because you can't have a 100% chance even after a million flips, just a 99.999999999999999% chance). Or if you had a 33% chance of hitting/flipping/drawing/rolling SOMETHING, you would have a 1 in 3 chance of hitting it. If you missed, probability just used up 33% of a 100%, so you have 67% left...33% of 67% is ABOUT 22% ... 33% + 22% is 55%. A 55% chance of hitting/flipping/drawing/rolling SOMETHING in two 33% hits/flips/draws/rolls. The best example is the flush draw. You have 9 outs, so 18% on the turn, 19% on the river. 100% - 19% = 81%, 19% of 81% is 16%...19% + 16% is 35%.

This math is hard to do everytime you are on the spot, my question is, is there a simpler formula (besides memorizing every % of every out, on the turn, the river, and the turn AND river?)?