Texas Hold'em Equity Calculator
The "Rule of 4 and 2" Is an Approximation — This Isn't
Most players learn to multiply their outs by 4 (with two cards left to come) or by 2 (with one card left) to estimate their chance of hitting a draw. It's a fast mental shortcut, and a good one, but it drifts from the true number as the out count grows. This calculator computes the exact hypergeometric probability alongside the rule-of-thumb estimate, so you can see precisely how much the shortcut over- or under-states your real equity.
The Formula
Estimate: Rule of 4 (two cards to come) or Rule of 2 (one card to come) = Outs × 4 or Outs × 2
Exact Equity vs. the Rule of Thumb
| Outs | Exact equity | Rule of 4 estimate | Gap |
|---|---|---|---|
| 4 (inside straight) | 16.47% | 16% | +0.47 pts |
| 8 (open-ended straight) | 31.45% | 32% | −0.55 pts |
| 9 (flush draw) | 34.97% | 36% | −1.03 pts |
| 12 (flush + straight draw) | 44.96% | 48% | −3.04 pts |
| 15 (combo wrap draw) | 54.12% | 60% | −5.88 pts |
The rule of thumb is fairly accurate up to about 8 outs, but overstates equity noticeably once the out count climbs into the mid-teens — exactly where the exact formula matters most.
Where This Gets Used
- Pot odds decisions — comparing your exact equity to the pot odds being offered is the core calculation behind every call-or-fold decision on a drawing hand.
- Studying away from the table — running exact numbers for common out counts (4, 8, 9, 12, 15) builds accurate intuition faster than relying on the shortcut alone.
- Multi-way pots — when several players are drawing, small equity gaps between the exact figure and the estimate compound and matter more.
How to Use This Calculator
- Enter Outs — the number of unseen cards that improve your hand to a likely winner.
- Enter Unseen Cards — typically 47 after the flop or 46 after the turn.
- Choose Cards to Come — 2 (flop to river) or 1 (turn to river).
- Select Calculate to see the exact equity and the quick rule-of-thumb estimate side by side.
Related Calculations
Check static hand-ranking odds with the Poker Odds Calculator, or apply the same hypergeometric logic to a different card game with the Solitaire Odds Calculator.