Rule of 2 and 4

Also known as: rule of 2 and 4, 2 and 4 rule, rule of two and four

A fast mental shortcut: multiply outs by 2 for one card to come, by 4 for two cards, to estimate draw equity.

The Rule of 2 and 4 converts outs into an approximate equity percentage without exact combinatorics:

• ×2 per card to come: outs from flop to turn, or turn to river.
• ×4 when you'll see both turn and river (all-in on the flop).

The ×2 multiplier is very accurate because one card is drawn from ~47 unseen (\(\tfrac{\text{outs}}{47}\approx \text{outs}\times 0.0213\), close to ×2). The ×4 version overcounts because it double-counts cards that hit on both streets. The fix: for large draws (roughly 9 or more outs), subtract a few points, or use the tighter rule \((\text{outs}\times 4) - (\text{outs}-8)\). For 8 or fewer outs, ×4 is reliable.

Use it live against pot odds: outs-to-equity, then check the price. It's an estimate, not a solver — fine for in-game decisions, but don't build thin river calls on the rounding error.