Calculating % will hit with 2 cards coming

[ur2god]

lets say your going from flop to turn
and you need 9 outs for your flush, so thats
like ~17% right? like 9*2-1 or whatever

but how do I calculate the % chance to hit
going from flop to river? its not 17%+17% even
thought thats pretty close. anyway thx!

[Razzputin]

The % chance of hitting is 1 - the chance of missing.
On flop you see 3 cards plus two in your hand.
So there are 47 unseen cards. If you have 9 outs then the chance that you will hit on the turn or river is (1- (38/47*37/46))
I usually just estimate by doing 2*outs with once card to come and 4*outs with two cards to come.

[jason_t]

Here is the derivation of that rule.