odds of hitting a flush

[Eric P]

In the back of HPAFP david sklansky says "we won't bother you with the math needed, but here is the odds to make your hand after the flop" or something to the like of that. For a flush draw (9 outs) he puts it at 2-1 against, or 33%. I was interested in how to get this figure exactly. If someone (even david or mason) could do one example for me i would be grateful.

[BruceZ]

There are 3 different methods in this post which give exactly the same result:

flush draw

Method 1 is most commonly used. You take 1 minus the probability of not hitting on either card.

Method 2 takes the probability of hitting on the turn plus the probability that you miss on the turn AND hit on the river.