Rule of four and two : common mistake

This is really one of my chief money makers in our live games, and thus very -EV explaining the mistakes I've seen many of you make with this seemingly easy to apply rule. Still, in the interest of improving everyone's game, let's take a look at the common mistakes a lot of you make.

Defination

The rule of four and two state that the chances of you winning on the hand is roughly equivalent to the number of outs you hold multiplied by 4 on the flop and 2 on the turn.

Easy enough to remember? Chances are, you have been applying this rule wrongly.

Let's start by understanding how we get 4 and 2 in the first place.


On the flop, there are 52-3-2 unknown cards. That's 47 cards that are mucked, burnt, dealt out or left in the deck. Since the cards are in a quantum state, the fate of each individual card doesn't matter. On the flop, there are two cards to come. that's 1/47+1/46 chance of hitting each of your outs. Add that together and you get roughly 4/100 = 4% per out. If you have 5 outs, your chances of winning by the flop is 4X5 = 20%. Simple enough. You can work out the rule of 2 yourself.

Let's take the most common misuse of this rule, the nut flush draw. What most of you have done really, when holding the nut flush draw is apply the basic concept of the rule. If 20 dollars is in the pot and someone bets the pot, you quickly apply the rule and decide you're getting correct odds at 1:2 to call the flop and see the turn. Now we know that calling a pot sized bet with a flush draw can't be correct, so where did the problem arise?

That's where most of you guys lose money. You took the rule and never bothered to understand how it worked. This rule only works if you are ALL IN on the flop or turn, respectively. You can't call 1:2 on the flop and 1:5 on the turn. That's only for all ins! You don't magically have twice the chance of drawing to the flush on the turn card than on the river. The double odds are because you are already all in and paying for TWO cards and not one.