Rule of 2 and 4
The rule of 2 and 4 estimates your chance of hitting a draw: outs times 2 for one card, outs times 4 for two.
With 8 outs on the turn, 8 × 2 gives 16%, and the exact figure is 17%. With 9 outs on the flop, 9 × 4 gives 36% by the river against an exact 35%.
The rule of 4 overstates big draws: 15 outs gives 60%, while the exact figure is 54%. Use the two-card number only when you will see both cards without paying again.
Work it out
Outs calculator: Enter how many outs you have and where you are in the hand. The calculator shows the exact chance of hitting, next to the rule of 2 and 4.
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Practise decisions like this in the trainer: ten hands at a time, with the answer and the reason.