- Maths reviewed by Josh Miller
- Tested by Diana Westfield
The rule of 2 and 4 is the bit of poker maths everyone learns first. Count your outs. On the flop, times 4 for your chance of hitting by the river. On the turn, times 2. A flush draw on the flop is 9 outs, so about 36%. The real answer is 35.0%, close enough to play with. The free CasinoCalcs calculator shows the rule, the exact number, and the gap between them, which grows faster than most people realise.
The rule of 2 and 4, and where it breaks
| Draw | Outs | Rule of 4 | Exact, flop to river | Rule of 2 | Exact, turn to river |
|---|---|---|---|---|---|
| Gutshot | 4 | 16% | 16.5% | 8% | 8.7% |
| Open-ender | 8 | 32% | 31.5% | 16% | 17.4% |
| Flush draw | 9 | 36% | 35.0% | 18% | 19.6% |
| Flush draw + overcards | 12 | 48% | 45.0% | 24% | 26.1% |
| Flush + open-ender | 15 | 60% | 54.1% | 30% | 32.6% |
Two things jump out. The rule of 4 overstates big draws, by six points at 15 outs. And the rule of 2 understates on the turn, because there are only 46 cards left, not 50.
The fix for big draws: times 4, then take off the outs above 8. Fifteen outs is 60 − 7 = 53%. Near enough.
Turning it into a decision
The rule only tells you how often you hit. To know whether to call, compare it with the pot odds. Say there’s $60 in the pot and villain bets $30. You’re calling $30 to win $120 with your call included, so you need 25% equity.
With a flush draw on the flop and both cards to come, 35% clears that easily. But the rule of 4 assumes you’ll see the river for free. If villain is likely to fire again on the turn, think one card at a time: 19%, which doesn’t. That’s the trap the rule of 4 sets for new players.
Counting outs honestly
An out only counts if it gives you the best hand. Your flush draw has nine outs, unless the board pairs and villain fills up, or one of your flush cards also gives him a straight. Overcards are worth less than they look. A common habit is to knock a couple off for “dirty” outs when the board is wet.
For full equity against a hand, use the poker odds calculator. To size up a call properly, the pot odds calculator and the implied odds calculator do the rest.
Good maths doesn’t stop a bad run. Keep poker money separate from bill money. BeGambleAware.org has free support if it stops being a game.