Quote Originally Posted by JeffreyGB
That said, when I am calculating, I use the (2/4 [flop/turn]) x outs formula mentioned above. It's easy and accurate enough to allow me to make my decision.
I use a slightly more accurate "easy" turn+river formula. It's' not exact either but much closer.

Outs * 4 - (# of outs over 8)

So 7 outs = 7*4 = 28%
8 outs = 8*4 = 32%
9 outs = 9*4-1 = 35%
etc....