post flop odds

It's easy enough to figure odds pre-flop in NL hold 'em, as long as you have an idea what your opponent has. With just the river to come, also pretty east. But I still struggle a bit with the post-flop odds. Any ideas on the easiest way to get these down pat? Your favorite book, a trick you learned, etc?

[jccookjr]

After the flop multiply your outs by 4 for your percentage to hit, then multiply by 2 after the turn. Will give you an approximate number.

[illegit]

Approximations

(Outs * 4) - (outs - 8) = Your % of hitting w/ 2 cards to come. Flush draw = 9 outs, so (9*4) - (9-8) = 35%

Outs * 2 = Your % of hitting with 1 card to come

[walleyes]

is this the formula Harrington refers to in HOH2

[abuljooj]

that formula is not very accurate. 9/47 = 19% which means that after the flop with 2 cards left to come, 9 outs = (19x2) = 38% not 35%. an easy way to figure all this out in your head is to take your number of outs, multiply it by 2 then add 1. 9x2=18+1=19. then multiply that by the number of cards left to come. this is the easiest most accurate way to do it aside from memorizing all the odds.