Doubling down is one of the few blackjack decisions that puts the maths on your side.
Basic strategy says to double hard 9 against a dealer's 3 through 6, hard 10 against 2 through 9, and hard 11 against almost any upcard. Soft hands, where an Ace counts as 1 or 11, carry their doubling rules from Ace-2 through Ace-7.
Permitting doubles is one way the house gives back some of the edge it collects by making players act first and risk busting. Most players who spend a little time with a basic strategy chart lock in the hard totals quickly. Soft hands are where even experienced players stay fuzzy, and they're where the clearest gains sit.
What Does Doubling Down Mean in Blackjack?
When you double down, you double your bet after seeing your first two cards. You then receive one card, no more and no less, and the hand is complete. You can't hit again and you can't back out.
Not every table allows soft doubling. Some restrict doubles to hard totals of 10 or 11, and some to hard 9, 10 or 11. More options are better for you, so the double down rules are one of the first things to check when you pick a table, whether you're at a live game or playing online blackjack.
When a basic strategy chart tells you to double, it's because the play carries a positive expectation. You won't win every one of those hands. Sometimes doubling means losing twice as much. Across thousands of hands, though, the average result of a correctly played double beats the average result of hitting or standing.
When Do You Double a Hard Hand?
Hard totals are the quick part. In six-deck games, three hands are worth doubling and nothing else is.
- Hard 11: double against every dealer upcard from 2 through 10. Against an Ace, double if the dealer hits soft 17 and hit if the dealer stands on all 17s.
- Hard 10: double against 2 through 9. Hit against a 10 or an Ace.
- Hard 9: double against 3 through 6. Hit against everything else.
Nothing below hard 9 doubles, because one card can't build a strong enough total. Nothing above hard 11 doubles either, because one card can bust you and you're stuck with the result.
Those three totals are the whole hard-hand list, which is why players working through the best blackjack strategies tend to have them memorised inside a session or two.
When Do You Double a Soft Hand in Six-Deck Games?
Soft hands are the ones with an Ace counted as 11, and they behave differently because a single card can't bust them. That changes which doubles pay off.
Two soft totals are never doubled in a six-deck game. Soft 19 and higher win more often than they lose when you stand, and taking a card only weakens them. Soft 12 is a pair of Aces, and the better play there is to split.
That leaves soft 13 through soft 18, or Ace-2 through Ace-7. The rules are:
- Ace-2 and Ace-3: double against 5 or 6. Otherwise hit.
- Ace-4 and Ace-5: double against 4, 5 or 6. Otherwise hit.
- Ace-6: double against 3, 4, 5 or 6. Otherwise hit.
- Ace-7: double against 2 through 6 if the dealer hits soft 17, or 3 through 6 if the dealer stands on all 17s. Otherwise stand.
Strategy for soft 13 through soft 17 is identical whether the dealer hits or stands on soft 17. Only soft 18 splits on that rule.
Every one of those plays raises your average return, and the margin widens as the dealer's upcard weakens.
Doubling Ace-6 against a 6 takes your average profit from 12.6 cents per dollar of your original bet to 25.2 cents on what is now a two-dollar bet. Against a 3, the same hand moves from 2.9 cents to 5.6 cents.
One case sits close enough to the line to be worth knowing. Ace-2 against a 5 returns an average of 13.8 cents per dollar if you hit and 14.1 cents if you double. Basic strategy still calls for the double, but the extra return you're buying with that second bet is thin, so it's the one hand on the list where a short bankroll makes hitting a defensible call.
Ace-6 and the Soft 17 Wall
Inexperienced players get stuck on soft 17. With a hard 17, basic strategy tells you to stand no matter what the dealer shows, and the habit carries over.
Soft 17 is a different animal. One card can't bust it, and a 17 only wins when the dealer busts. At best it pushes a dealer 17. Standing gives up the upside and buys you no protection in return.
Players who know their basic strategy never stand on soft 17. They hit it, and they double it when the dealer shows 3, 4, 5 or 6. The dealer's version of the same hand is worth understanding too, because whether the house hits or stands on soft 17 changes one of the plays below.
Ace-7 and the Dealer's Soft 17 Rule
Soft 18 is the one soft total where the dealer's rule changes your play. It's also the one where the alternative to doubling is standing instead of hitting, since a soft 18 is already a hand worth keeping.
In a six-deck game where the dealer hits soft 17, double Ace-7 against 2 through 6. When the dealer stands on all 17s, the dealer no longer gets a chance to turn a 17 into something better, which creates extra hands your soft 18 already beats. That shifts Ace-7 against a 2 from a double to a stand.
The numbers for Ace-7 against a 2 in a stand-on-soft-17 game: standing averages 12.3 cents per dollar, doubling 12.1 cents and hitting 6.3 cents. The gap between standing and doubling is small, and standing gets you the better average without putting a second bet at risk.
What Changes in a Single-Deck Game?
Single-deck games are uncommon, and the ones you do find usually carry rules elsewhere on the felt that shift the blackjack odds back towards the house. If you sit at one, the soft doubling rules tighten:
- Ace-2, Ace-3, Ace-4 and Ace-5: double against 4, 5 or 6. Otherwise hit.
- Ace-6: double against 2 through 6. Otherwise hit.
- Ace-7: double against 3 through 6. Otherwise stand. There's no double against a 2 here, whether the dealer hits or stands on soft 17.
- Ace-8: double against 6. Otherwise stand.
Ace-8 is the one that catches people out, because soft 19 is never doubled in a six-deck game. In a single-deck game with the dealer hitting soft 17, standing on Ace-8 against a 6 averages 44.7 cents per dollar and hitting averages 23.3 cents, while doubling returns 46.7 cents.
When the dealer stands on all 17s, the figures are 48.24 cents standing, 24.1 cents hitting and 48.26 cents doubling. That last margin is thin enough that skipping the second bet is defensible, though the long-run average is highest if you take it.
Can You Double a Three-Card Hand?
Standard blackjack allows doubling on your first two cards only. Once you've taken a hit, the option is gone, so a soft 16 built from Ace-2-3 or a soft 18 from Ace-3-4 gets played out with hits and stands.
The rules for those hands are short:
- Soft 17 or lower: hit.
- Soft 18: stand against a dealer 8 or lower, hit against 9, 10 or Ace.
- Soft 19 or higher: stand.
As with every basic strategy play, these won't win every time. They give you the best average result over the long run.
Key Takeaways
- Doubling down doubles your bet and buys exactly one more card, with no option to hit again afterwards.
- Hard hands: double 9 against 3 through 6, 10 against 2 through 9, and 11 against 2 through 10, plus against an Ace when the dealer hits soft 17.
- Soft hands in six-deck games: Ace-2 and Ace-3 against 5 or 6, Ace-4 and Ace-5 against 4 through 6, Ace-6 against 3 through 6, and Ace-7 against 2 through 6 when the dealer hits soft 17. Otherwise, hit, except with Ace-7, where you stand.
- Never double soft 19 or higher in a six-deck game, and never double soft 12, which is a pair of Aces and should be split.
- Single-deck play tightens the low soft hands to 4 through 6 and adds a double on Ace-8 against a 6.
- You can only double on your first two cards, so a soft hand with three or more cards is played with hits and stands.
