05- 5-09, Clark Jensen:
What to Think of When Drawing to a Flush
Yes, there is a way to figure this out, but to be totally honest you don't need to know how it is done as long as you know the odds for hitting the most common draws. If you have a flush draw on the flop, the chance of hitting the flush on either the turn or the river is about 1 to 2 (35%). If you have a flush draw on the turn, the odds are about 1 to 4 (19%).
These figures derive from comparing how many outs you have (cards that help you) with the number of cards that don't improve your hand. If you like to know how this calculation is done, read Drawing to Turn or River Odds.
It doesn't matter how many players are in the hand since you just view all cards remaining in the deck and your opponents' hole cards as unknown. If you have two spades, and there are two spades on the board, there are 9 spades left in the ocean of unknown cards. Since you can't possibly know what the other players are holding, this is the only information you have to base your decisions on. But if you allow me to leave the math for just a moment things can become a little different.
In a real game you get more information than just the odds. Say that the board is Ac - Tc - 8s and you have a small flush draw with 2c - 4c. A player bets out, you call, and two players behind you also call. If you calculate the odds, there is a 35% chance that you hit the flush on turn or river, but that is NOT the same as you getting the best hand. You could catch a perfect card just to be beaten by a higher flush. And the more players that are in the hand, the greater the risk is that someone else is on the same draw as you.
So, does it matter how many players are in the hand when calculating odds? No. Does it matter when you decide what to do? Yes. Knowing the odds gives necessary when making decisions in poker but there is other vital information to take into consideration as well.
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