The greatest tool a blackjack player has is the ability to increase a bet AFTER the cards are dealt. To win at blackjack, you must double down when the odds of winning are in your favor — but only when they are.
When You Have 11
This is the best chance for a big win. You have a 4-in-13 chance of making 21, and another 4-in-13 chance of completing a strong hand (17–20). In 8 of 13 draws, you significantly strengthen your hand.
Rule: Double down against everything but a dealer's Ace.
Since your 3-card 21 can only be beaten by a natural blackjack, the risk is minimal.
When You Have 10
You are very likely to pull a strong hand — especially a 20. You want to ensure the dealer cannot make a blackjack.
Rule: Double down only against a dealer's 2 through 9.
Against a 10 or Ace, the dealer has too strong of a starting position.
When You Have 9
With 9, you're not looking for a strong hand — you want to take advantage of a dealer's weak hand and increase your bet when the dealer is likely to bust.
Rule: Double down only against a dealer's 3 through 6.
Avoid the aggressive play of doubling on 9 against a dealer's 7 or 8.
Card Counting Adjustments
These rules assume no card counting. If you are counting cards and the deck is loaded with high cards, there are additional situations that warrant doubling — particularly when you have 8 or 9 and the remaining deck heavily favors high cards.
The Bottom Line
Correct doubling strategy alone can reduce the house edge by approximately 1.5%. Combined with basic strategy and card counting, you have a significant edge over the casino.