Odds, equity and theory
Rule of 2 and 4 in poker
Definition
A shortcut: multiply your outs by 2 for one card to come or by 4 for two cards to come to estimate your chance to hit, in percent.
The rule is accurate for a few outs and overestimates with many: with 9 outs the rule of 4 says 36% while the exact figure is about 35%. For 15 or more outs, subtract a few percent from the rule of 4.
Use it at the table to compare your chance to hit with the equity needed from the pot odds.
Example
A flush draw with 9 outs after the flop is about 9 × 4 = 36% to hit by the river, or 9 × 2 = 18% on the next card.