The greatest tool a Blackjack player has is the ability to increase a bet AFTER the cards are dealt. Can you imaging betting on a football game and being allowed to double your bet in the 3rd quarter if you want?
To win at Blackjack, you must Double Down when the odds of winning are in your favor but only when the odds are in your favor. Some novice players are over-aggressive and double on 7s and 8s against a dealer’s 6. Others are too passive and only double on 11 against a dealer’s 9 or less. Neither strategy will make you profitable at Blackjack in the long run but using the following strategy can. Again, these suggestions are the result of simulations of millions of Blackjack hands and although they will not be right every time, they will help you lose less and win more.
When you have 11:
This is the best chance for a big win as you have a 4-in-13 chance of making 21. There is also a 4-in-13 chance of completing a strong hand (17-20) so in 8 of the 13 draws you greatly strengthen your play. The other 5/13 of the time (12-16) you can still win if the dealer busts. Since your 3-card 21 can only be beaten by a natural Blackjack you should double down against everything but a dealer’s Ace.
When you have 10:
Now its tough to get 21 but you are very likely to pull a strong hand, and especially a 20. You want to be sure the dealer cannot make a Blackjack and so you should double down only against a dealer’s 2-9.
When you have 9:
With 9 you are not looking for a strong hand but rather you want to take advantage of a dealer’s weak hand to increase your bet when it is likely he will bust. Many novice players like to Double on 9 against 7 or 8 but to win you need to avoid this aggressive play and you should double down only against a dealer’s 3-6.
These rules assume that you are not counting cards. If you are counting cards and come across situations where the deck is loaded with (or depleted of ) face cards there can be some other situations that warrant consideration but they require too much analysis to discuss here.