Doubling down is when the player opts to double his/her wager after being dealt the first two cards in a hand of blackjack. The player then receives only one additional card before the hand is finalized. This technique has been touched upon in other places, but has never been fully discussed. Use this article as a companion to Blackjack: Basic Strategy and Should I Always Split Pairs in Blackjack?
From the description above, it should be clear that the player doubles down only when adding a single card to his/her hand can significantly increase its strength. This situation would happen with a starting hand of either ten or eleven, where adding a card with a value of ten would make 20 and 21, respectively. Thus, whenever you dealt a starting hand of ten or eleven, you should always double down unless the dealer has a ten or an ace showing, though with an eleven you should double against the ten as well. As mentioned in the article about splitting, a pair of fives is treated the exact same way as any other hand of ten. Additionally, when you have a starting hand of nine, you should double down if the dealer’s up card is anything between three and six.
While these situations may be somewhat obvious, there are several other times when one should double down at the blackjack table; namely, these occur when the player has a soft hand and the dealer has a relatively weak hand (an up card between three and six). With a soft 17 or 18, you should always double down in one of these situations. Although it may seem counterintuitive to break up a hand as strong as 17 or 18, which you would normally stand on, having an ace in your hand allows you to hit with impunity, since you can never bust. Couple this with a bust-friendly hand for the dealer and your expected value for doubling down is higher than any other decision.
Keeping the above information in mind, the player can now understand why it is profitable to double on the following hands: On a soft thirteen or fourteen, double down only if the dealer is showing a five or a six, the two cards most likely to result in a bust. With a soft fifteen or sixteen, add a four to the hands on which you may double down against. Note that these rules apply to games in which the dealer stands on soft 17 (recommended games to play in due to the lower house edge); when the dealer can hit on soft 17, you must increase your doubling range slightly on soft 18 and 19 as well as doubling down on any hand in which you start with eleven.