about 1 in 12 or 8.21%
The probability of hitting your flush draw with two suited cards on the flop is 1/50/12 x 1/49/11 or approximately 1 chance in 18.5 tries multiplied by 100% plus 11/48 or a total percentage probability of 8.21% or about 1 in 12 attempts.
I did not know the answer myself I am a mathematics major currently in precalculus. I did the math on a TI-84 plus scientific calculator. First you must understand what is being calculated. 2 of 3 cards must be your suit on the flop. 50 cards remain in the deck on the 1st card and 12 of your suit remains in the deck. 1 less of each on the 2nd card. The 3rd card is a multiplier because you only need 2 out of 3, not 3 out of 3. So now the probability goes up not down!