Poker Basics - The Rule of 4 & 2

A tried and tested short hand method for working out your odds of hitting your draw on the turn and river.

One of the first introductions to the math of poker many players experienced is the Rule of 4 & 2, which is a simple heuristic that still holds up today and very important for online poker.

It is a simplified method of calculating your odds of hitting a draw on the turn or river. Quite simply if you have a drawing hand on the flop, the odds of you hitting your made hand is the number of outs you have multiplied by 4 when you have two cards to come, or multiply them by 2 if you have one card to come. 

So let's say you have K 9 on a flop of A 6 3, if you hit your heart by the river you will have the best hand. There are nine hearts left in the deck, so if you see both the turn and river, multiply 9 by 4 and you get 36%. You roughly have a 36% chance of hitting your flush. 

If the turn brings a 2 then there is only one card to come, now you multiply your outs by 2. 9 x 2 = 18%. You are now 18% likely to hit your flush on the river. 

The Rule of 4 & 2 is most useful when you are considering calling a bet or making a semi bluff. In our original example if the pot is $100 and your opponent goes all-in for their remaining $66, a quick calculation will tell you that you only need 29% equity to break even here. Since you are 36% to hit the nuts and there is also a slim chance hitting a pair might win it for you, that makes this an easy call. 

Likewise, when you know you have 36% equity against any hand, that makes semi bluffing very enticing here. You could go all-in if you think your opponent is likely to fold a decent amount of the time and if they don't you still win 36% of the time. 

Do you have clean outs?

Some important things to consider to stop you misapplying the Rule of 4 & 2. 

First of all, the 4 rule only works when you are guaranteed to see both turn and river cards. So for example if your opponent goes all-in. It does not work well if you call a bet on the flop but are likely to fold the turn if you miss. In that respect, the heuristic of multiplying your outs by 2 is better for each street. Your odds are always slightly better on the river alone than the turn alone because the turn card removes one 'unblocker' from the deck.

Secondly, you have to be confident you have clean outs that will definitely give you the best hand when you hit. If, for example, you had the T 7 in the hand above, you only might get action by a better flush when you do hit. Then, of course, some turns might bring a card that would make hitting your hand a disaster, for example if the turn was a 6 it would pair the board making a potential Full House that renders your flush useless. 

As a general rule to use in real time the Rule of 4 & 2 is very useful. Just check out what the actual odds of hitting some common draws are below to see how accurate it is (the Rule of 4 & 2 shorthand odds are in blue). 

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