Why Does the House Edge in Blackjack Increase with the Addition of Decks?
Playing Blackjack with Multiple Decks
The house edge in blackjack increases when the house uses multiple decks. Savvy blackjack players search for single deck blackjack games, but most casinos deal blackjack from "shoes" that contain as many as eight decks of cards. Having these additional decks of cards increase the house's edge, but why?
The reason for this change in math is because the additional decks decrease the chances of a player getting dealt a natural. (A natural is an initial total on two cards of 21, which pays out at 3 to 2.) Since there are more cards in the deck, the effect of the tens and aces is diluted.
Think about it this way. Suppose you're playing in a single deck blackajck game, and you're dealt a ten. Your chances of getting a blackjack are the same as your chances of being dealt an ace on the next card. That's an easy probability to calculate. There are 51 cards left in the deck--you've been dealt one of them already. Four of those cards are aces, so your chance of getting an ace on that next card is 4/51, or 7.84%.
Now suppose you're playing in an eight-deck game, and you're dealt a ten. There are 32 aces left in the deck, but now, the deck consists of 415 additional cards. Your chances of being dealt an ace are 32/415, or 7.71%. That seems like a small difference, but the casino is working on a mathematical model that deals in millions of hands. A tenth of a percent is a big deal over the course of a million ten dollar bets.
More importantly for the casino, using these additional decks makes counting cards harder. I explained counting cards in yesterday's post, but I think it's easy to see that the effect of a single ace out of 52 cards is dramatically different from the effect of a single ace out of 416 cards.
Card counters compensate for this by converting their "running count" into a "true count." Basically, this means dividing the tally that you're keeping mentally by the number of decks in play. If you would normally double your bet when the count is +1 in a single deck game, you'd only double your bet in an eight-deck game when the running count becomes +8. And guess how much more likely profitable situations are in a single deck game than in a multiple deck game.
Of course, in some casinos, they'll use continuous shuffling machines with large decks of cards in order to eliminate the edge a card counter might obtain. If the cards are constantly being mixed up, then what happened in previous hands no longer has any effect on the hand you're playing.
And that's why the house edge in blackjack increases when you're playing with more decks.