[olavfo]

Welcome to the forums.

Let's say you have a backdoor flush draw with two spades on your hand and one spade on board. You need spade-spade on turn and river.

Before the turn there 47 unknow cards and 10 spades. If a spade comes on the turn, there are now 46 unknown cards and 9 spades.

The chance of hitting spade-spade is

P = (10/47)*(9/46) = 90/2162 = 0.0416 = 4.16%

In odds form:

(1/P -1) : 1 = (1/0.0416 - 1) : 1 = 23 : 1

So the math is simple. Calculate the chance of hitting on the turn and multiply it with the chance of hitting again on the river. (Remember to adjust the number of outs and unknown cards on the river. You have one less out, since the turn card hit, and you also have one less unknown card.)

olavfo