The Rule of 2 and 4, and its error
A shortcut that is good to about twelve outs, and how far it drifts after that.
4 minute read
Multiply your outs by 2 for the chance of hitting on the next card, or by 4 for two cards to come. Nine outs: 18% and 36%, against exact figures of 19.15% and 34.97%. Close enough for any decision at the table.
The shortcut is linear; the real formula is not. Past about twelve outs the Rule of 4 starts overshooting: fifteen outs is 54.1%, not 60%. A common patch — subtract (outs − 8) from the Rule of 4 result when outs exceed eight — brings fifteen outs to 53%.
The 21 highlighted cards are your outs.
What you’re drawing to
- Flush draw9 outs
- Open-ended straight draw8 outs
- Two overcards6 outs
Pairing an overcard makes top pair, which is not always the best hand.
- Combo draw: a flush draw and a straight draw at once. Shared outs are counted once.
Shortcut vs. exact
| Cards to come | Rule of 2/4 | Exact | Gap |
|---|---|---|---|
| Turn only | 42% | 44.68% | −2.68 pts |
| Turn and river | 84% | 69.94% | +14.06 pts |
Quick check
Where does the Rule of 4 start to be more than 3 points off?
Show answer
Around 13 outs and above; at 15 outs it says 60% against an exact 54.1%.