Poker Equity & OddsOpen-ended straight draw odds
Short answer
An open-ended straight draw has 8 outs and makes the straight by the river 31.5% of the time from the flop, or 17.4% from the turn with one card to come. The rule of 4 and 2 says 32% and 16%, close enough at the table. A gutshot has half the outs: 16.5% from the flop, 8.7% from the turn. The numbers are counted by the Poker Equity & Odds engine.
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Eight outs and four outs, exact
| Draw | Outs | Flop: next card | Flop: by the river | Rule of 4 | Turn: river | Rule of 2 |
|---|---|---|---|---|---|---|
| Open-ended straight draw | 8 | 17.0% | 31.5% | 32% | 17.4% | 16% |
| Gutshot | 4 | 8.5% | 16.5% | 16% | 8.7% | 8% |
The flop number is a bit less than double the one-card number, because once the turn hits, the river doesn't add anything. We also counted it the long way: of the 1,081 possible turn and river pairs, 340 contain an eight or a three, which is the same 31.5%. How often a connector flops this draw in the first place is on the odds of flopping a straight page.
On a real hand
Against A♦ 7♣ the draw has 34.2% on the flop, over all 990 turn and river pairs. That is a bit more than the 31.5% hit chance, because a few boards win without a straight, like a five and a four for two pair. Put the K♣ on the turn and it is 18.2%: 8 of 44 rivers. Why 44 and not 46? Here you know his two cards and neither is an out, so the same 8 outs sit among fewer unseen cards.
Next to a gutshot
open-ender, turn K♣
8 outs against A♦ 7♣.
gutshot, turn 3♦
4 outs against K♠ J♦.
9♣ 8♦ on J♥ 7♠ 2♣ is drawing to a ten. Against K♠ J♦ it has 20.3% on the flop and 9.1% after the 3♦. On one card the open-ender is exactly twice as likely to get there, 8/46 against 4/46. From the flop it is a bit less than twice. Whether either one is worth a call depends on the bet, and should you call with a draw has that table.
When the outs are not clean
Same five-four, but now the board has two diamonds and he holds A♦ 7♦. The 8♦ and the 3♦ still make your straight, and they make his flush too. The draw drops from 34.2% to 20.3% on the flop, and after the K♥ only 6 of 44 rivers win, 13.6%. If you think he has the flush draw, count six outs, not eight. On some boards the top end is dirty too, because the card that makes your straight gives a bigger one to anyone holding the card above it.
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Questions
How many outs does an open-ended straight draw have?
Eight: four cards at each end. 5♣ 4♣ on 7♦ 6♠ 2♥ makes a straight with any eight or any three.
What are the odds of hitting an open-ended straight draw?
31.5% by the river from the flop and 17.0% on the turn alone. From the turn, with one card to come, it is 17.4%.
Is an open-ender as good as a flush draw?
A little worse on outs: 8 against 9, so 31.5% against 35.0% by the river. The flush draw guide has its numbers.
How much worse is a gutshot?
About half. Four outs hit 16.5% by the river from the flop and 8.7% from the turn, against 31.5% and 17.4% for the open-ender.
Where the numbers come from. Every figure on this page comes from the app's own engine, the code that runs on your phone. Outs come from the Outs & Odds tool, hit chances are exact counts, and equities count every runout with no sampling. Engine build 482550e8. More worked numbers: flush draw odds and the pot odds cheat sheet.