Various Numbers of Decks (Blackjack)

Fundamentals of Blackjack by Chambliss and Roginski

Of vastly greater importance than uncommon bonus hands are the effects which arise from using various numbers of decks. As we have already stated, a single-deck game is significantly more advantageous to the player than is a multiple-deck game which uses the same rules. But why is this so? There are several reasons. Let us compare a single-deck game with the eight-deck shoe game which now prevails in Atlantic City. In a single-deck game the likelihood of a player being dealt a Blackjack is 1/20.72. In an eight-deck game it is 1/21.07. What is greatly increased in the latter case, however, is the probability that both the dealer and the player will have Blackjack in the same round. In the single-deck game this is 1/27.22, but in the eight-deck game this is increased to 1/21.71. If you have Blackjack and the dealer doesn’t, you are paid off at 1.5 times your original wager. But if the dealer also has Blackjack, it is a standoff and you win no money. Since Blackjack pushes are 20% more likely in the eight-deck case than with single decks, it is clear that your earning potential will be reduced when multiple decks are used.

Another significant adverse factor in a multiple-deck game is the fact that the player’s expectations for most doubling situations are significantly reduced from what they are in the single-deck case. A good hand suitable for doubling such as 10 or 11 usually consists of two relatively low and undesirable cards. Their removal from the deck increases the likelihood of getting a good hit. Consider the case of doubling on 11 in a situation in which the dealer’s up card is other than a ten or ace. In the single deck case the probability of reaching 21 is 16/49 or 0.3265. In the eight-deck case this is reduced to 128/413 or 0.3099. Thus, a player in the latter game is 5% less likely to reach 21 by doubling on 11 than he is if he is playing a single-deck game.