In blackjack, doubling your bet is usually only allowed when your hand totals 9, 10, or 11. However, some casinos allow players to double down on any two-card hand, and a few even allow it after splitting pairs.
When the dealer shows a low card (2 through 6), he has a better chance of busting than when he shows a high card (7 through ace). Therefore, it makes sense to double your bet when the dealer is likely to bust.
That’s why the blackjack doubling rule is usually restricted to hands of 9, 10, or 11.
Of course, you don’t always have to wait for the dealer to show a low card before doubling your bet. If you have a hand of 11 and the dealer shows a 10, you should double down regardless of the dealer’s up-card.
And if you have a hand of 10 and the dealer shows a 9 or lower, you should also double down.
The same goes for pairs of 8s and 7s against dealer 2 through 7; and pairs of 6s against dealer 2 through 6. In all these cases, the math favors doubling down.
There are exceptions to every rule, however. If you have a hard 16 (a hand made up of two 8s, for example) and the dealer shows a 9 or higher up-card, you should stand rather than double down.
The same goes for hard 15 against a 10 up-card; and hard 13 and 14 against an ace up-card.
The bottom line is that doubling down is only worthwhile when you have a good chance of winning the hand by doing so. When the dealer is likely to bust, it makes sense to double your bet; but when the odds are stacked against you, it’s best to play it safe and stand pat.