Open-ended straight draw odds
What are the odds of hitting an open-ended straight draw? The short answer is below, and under it the working: the formula with the numbers put in, and the count from dealing every possible card.
Why it is 8 outs
9-8 on 7-6-2 needs a ten or a five, at either end. Four of each rank are unseen: eight outs. A double gutshot (9-7 on J-8-5, needing a ten or a six) is also eight outs and plays identically, and it is harder for an opponent to read.
The working
On the flop you have seen five cards, so 47 are unseen. They come off in pairs, turn and river, and there are C(47, 2) = 1,081 ways for that to happen. You miss only when both cards are among the 39 that do not help:
P(miss) = C(39, 2) / C(47, 2) = 741 / 1,081 = 68.55% P(hit) = 1 − 68.55% = 31.45% (2.18 to 1 against) Turn = 8 / 47 = 17.02% River = 8 / 46 = 17.39% (if the turn missed)
Then the engine dealt every one of the 1,081 runouts to the example hand and counted how many finish with a straight or better: 340 / 1,081 = 31.45%. The same count as the formula, runout for runout, so on this board nothing else gets you there.
Is it a call?
Calling a bet of B into a pot of 1 risks B to win 1 + B, so you need B ÷ (1 + 2B) of the final pot. The table compares that with the draw's chance of arriving. “All-in now” means you see both cards for one price; “one card only” means there will be another bet on the turn, and then the draw has one card at a time. Where one card is not enough, the right-hand column says how much extra you must win on the hands where it hits (implied odds) to make the call break even.
| Bet faced | Equity needed | All-in now (31.5%) | One card only (17.0%) |
|---|---|---|---|
| ⅓ pot | 20.0% | Call | Fold, or win 0.29× pot more when it hits |
| ½ pot | 25.0% | Call | Fold, or win 0.94× pot more when it hits |
| ⅔ pot | 28.6% | Call | Fold, or win 1.58× pot more when it hits |
| Pot | 33.3% | Fold | Fold, or win 2.87× pot more when it hits |
| 1.5× pot | 37.5% | Fold | Fold, or win 4.81× pot more when it hits |
This assumes every out wins and nothing else does: if your opponent can make a better hand on the same card, those outs are dirty and the real equity is lower. The spot calculator prices it against a whole range instead of a single hand.
Missing that? The gutshot is half the outs and a lot less than half the value, and with a flush draw as well it is a combo draw.
Hitting is half of it; the other half is the price. The pot odds pages show which draws can call a third-pot, half-pot or pot-sized bet, and what each one must win later when it cannot.
Pot odds by bet size →More drawing: turn and river
- Flush draw (9 outs)35.0%
- Gutshot (4 outs)16.5%
- Combo draw (12 to 15 outs)54.1%
- Two overcards (6 outs)24.1%
- Set to full house33.4%
- The rule of 2 and 4, checkedup to 12.5 pts off
Other odds
- Counted, not quoted.
tools/build-odds.mjsdeals every possible flop, turn, river or starting hand and counts. Where a closed-form formula exists, the build refuses to publish unless the count matches it exactly. - Made hands come from the engine. Whether cards make a straight, a flush or a full house is decided by the same Rust/WebAssembly evaluator the trainer runs, not by hand-written rules.
- Odds are not equity. Hitting your hand is not winning the pot. For that, the matchup pages deal every board against a specific hand and the spot calculator works against a range.
- Raw data: odds.json.