This post will be going over flush draw theory and how to play accordingly.  First things first, we have to do some math to understand everything fully.

First, let’s define what I mean by flush draw.  A flush draw is when there is the river or the turn and river to come, and you have four out of the 5 cards needed to make a flush.  The most powerful flush draw is going to be when you have both the turn and river to come, as you have two chances to hit, versus one if you only have the river left.

The most common situation is when you have two suited cards in your hand plus two more suited ones on the board.  Because of this and the fact that it becomes super situational if you have one in your hand and three on the board and therefore very difficult to talk general strategy, I am going to stick to the two and two scenario going forward.

The math on if you have a flush draw on the flop is as follows:

There are 13 cards of the same suit in a deck.  That means there are 9 remaining in the deck that will complete your flush draw.  After the flop, there are 47 remaining cards that are unknown to you.  That means the odds of the next card being one that completes your flush is 9/47.  If it doesn’t come on the turn then there is now one more known card to you.  The probability that it comes on the river is 9/46.  So we now know the probability of a card coming that completes your flop is ~19.1%, on the river ~19.6% and on either the flop or the river ~35.0%.

These percentages are really important going forward to determine whether chasing your flush card is worth it or not.

Generally speaking, you complete your flush draw just over a third of the time, if you have a four-card flush draw on the flop.  This means that if you get 3:1 pot odds (the amount of money you must call compared to what you can win) or better for it makes sense for you to call.  Well not always.  This all has to do with creating positive expected value.  If you create positive expected value then you will make money over the long hall and vice versa if negative.

So, if you can guarantee that there will be no betting on the turn then yes calling with 3:1 pot odds makes perfect sense.  Two common situations that this might arise in would be all ins or if you know it will be checked to you in position allowing you to check and end the betting.

But if you are in a situation where you will be facing another bet on the turn, then you need to get better than 3:1 pot odds.  Here is why.  If you miss on the turn, then the chances you hit on the river are just under 20%.  If you face a bet after the turn you now need 5:1 to be able to call.  Otherwise, you have negative expected value.  So most of the time if you do not hit on the turn you will fold, as most bets are larger than 5:1 pot odds.  If you don’t see the river then going back to your call on the flop, your percentage of hitting is under 20%, not 35%.  Your pot odds needed to be 5:1, not 3:1.

In conclusion, if you can guarantee that you will not face a bet on the turn, then you can call with 3:1 or better pot odds.  If you think you will face a bet, then you need 5:1 odds.