Why Do More Decks Increase the House Edge in Blackjack?

If you play blackjack, you’ve probably heard that fewer decks are better for the player. And if all other rules are equal, that’s true. The house edge is lowest on single-deck games, and increases with each deck added.

A  player emailed me to  ask, “Why does adding decks increase the house edge?”

It’s all in the math. If you start your hand with an Ace, 16 of the other 51 cards, or 31.4 percent, are 10  values that will give you a blackjack. If there are six decks in play, 64 of the other 311 cards, or only 30.9 percent, are 10 values. More blackjacks are dealt with fewer decks, and since players are paid 3-2 on those hands at most tables, that favors players.

Similarly, if the dealer has an 10-value face up in a single-deck game, four of the other 51 cards, or 7.8 percent, are Aces. In a six-deck game, 24 of the remaining 311 cards, or 7.7 percent, are Aces. Again, more blackjacks are dealt with fewer decks, and that’s in the players’ favor.

With fewer decks, you’re more likely to draw a 10 in double down situations. If you have 6-5 in a single-deck game, 16 of the other 50 cards, or 32 percent, are 10 values that will give you 21. In a six-deck game, it’s 96 of 310 cards, or 31 percent. You’ll get that wished-for 10 on your double downs more often with fewer decks.

Still, when you’re choosing a game, watch out for that “if all other rules are equal” condition. Rules on the dealer hitting soft 17, limiting double down situations paying only 6-5 on blackjacks can leave single-deck games as weaker plays than their multideck counterparts.

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