You’ve heard it time and time again – the fewer decks used in a game of blackjack, the better your odds.

Have you ever stopped to wonder why this is?

I got the inspiration to write this blog post after fielding a question from a buddy of mine who isn’t much of a gambler. He’s preparing to head to a bachelor party in Vegas and started to study blackjack. He noticed that everyone says “fewer decks is better,” but didn’t understand. After all, he said, the overall proportion of each rank of card is the same no matter how many decks are shuffled together.

This post is all about deck size and blackjack. We’ll cover why fewer decks is better, in detail, and include a little bit of math where appropriate.

**Fewer Decks = More Blackjacks**

The main reason we say that fewer decks is better for players is that, in each deck, exactly 1/13^{th} of all cards are Aces.

Yes, my friend was right, the initial proportions of card values to one another is equal no matter how many decks you play with. The reason you’ll be dealt more blackjacks with a smaller shoe is that the impact of removing a card from the game is greater in a game with fewer overall cards.

**Odds of Drawing Blackjack in a Single-Deck Game**

Let’s start by getting an idea of how often a player will draw blackjack in a single-deck game. To get to the probability of drawing a blackjack from a one-deck shoe, all you have to do is multiply the odds of drawing an Ace by the odds of drawing any card with a value of ten points. We know that a single deck of fifty-two cards contains four Aces and sixteen cards worth ten points – four tens, four Jacks, four Queens, and four Kings.

That means probability of drawing any Ace is 4/52, which we simplify to 1/13. Once you’ve drawn your Ace, the probability of then drawing any ten-point card is 16/51. Notice anything about those two numbers? The first probability is based on a fifty-two card deck, but since you’ve already drawn a card, you have to now work out the probability of drawing one of sixteen ten-point cards from a deck of fifty-one.

This change in the divisor is the reason why a smaller number of decks is advantageous to the blackjack player, and gives the house a distinct disadvantage.

If you want to get an accurate number of the likelihood of drawing a blackjack from a single-deck shoe, you actually need to double your result, since you could technically get a blackjack with either a ten-point card OR an Ace at the start.

All told, the probability of drawing a blackjack from a single deck shoe is 4.83%. That’s the probability of drawing an Ace (1/13) multiplied by the probability of drawing any ten-point card (16/51), multiplied by two.

**Odds of Drawing Blackjack in a Two-Deck Game**

To give you an idea of the statistical difference between one and two decks, let’s look at the odds of drawing a blackjack when you start with 104 cards instead of 52.

The probability of drawing any Ace from a two-deck shoe is 8/104. The probability of then drawing any ten-point card from the same shoe is 32/103. When we multiply those two together, then double the result, we get 4.78%.

Odds of drawing a blackjack in a one-deck shoe = 4.83%. Odds of drawing a blackjack in a two-deck shoe = 4.78%. By adding a deck (and not changing any rules of the game), the casino has decreased your likelihood of drawing a blackjack by 0.05%. Remember, though, that each one of those lost blackjacks would have been a win under standard casino rules at a 3:2 payout. Losing those 3:2 payouts makes a big impact on your bottom line, and on the casino’s.

**Why do Double Downs Work Better with Fewer Decks?**

If you’re following along closely, you’ve probably already figured out that the same phenomenon that makes blackjacks more likely with fewer decks probably also affects the likelihood of a successful Double Down. If you double your initial hand (6 and 5), you’ll be more likely to draw a face card to form a total of 21 if the game uses fewer decks.

Here’s where things get tricky – don’t forget that your dealer also benefits from these changes at lower deck counts. It’s not just the player that has a shot at more blackjacks. The reason why this fact doesn’t impact the game as much as it might is that players win 3:2 for blackjack, while the house wins just even money. Also, the dealer can’t Double Down, while the player can. That additional doubled-win gives the player a bigger advantage than the dealer.

**Conclusion**

All things being equal, meaning all rules being pretty much identical, a blackjack game that uses a smaller number of decks to build the shoe is advantageous for the player. One situation I’d warn blackjack players about – casinos that offer single-deck blackjack with a 6:5 or even 1:1 payout for player blackjack. The implication is that the casino is only willing to give you those improved single-deck odds in exchange for a reduced penalty at the point of player blackjack. In the case of games that don’t pay the traditional 3:2, don’t play them just because of the appeal of the single deck setup.