If you've read anything about blackjack, you've read this—always look for a blackjack game with fewer decks rather than more decks. But maybe you don't understand why the number of decks matter. Explaining that is the purpose of this post.
Here's why the number of decks matter, in a nutshell:
Fewer decks mean you have a better chance of winning.
You're more likely to be dealt a natural when you have fewer cards in the deck. This makes doubling down more effective. But it also means you'll see a 3 to 2 payout that much more often.
Here are the odds of getting a natural according to the size of the deck:
You have a 4.83% chance of getting a blackjack in a single deck game.
That drops to 4.78% if you're playing in a game with two decks.
Bring the total up to six decks, and your odds of getting a natural drop to 4.75%.
That doesn't seem like a big difference, but the payout of 3 to 2 on that hand makes up so much of the player's expected value (EV) that it's hard to underestimate the effect of losing even a few hundredths of a percent.
But you don't just make more money when you get dealt a natural. You also get more blackjacks when you double down on an ace. 3 to 2 is a great payout, but it's an even better payout if you double down first.
The dealer also gets more naturals when you're playing with fewer decks, but that's not as big a deal, because the dealer doesn't get a bonus win from you when she gets a blackjack. You just lose unless you have a natural, too.
Dealers don't get to double down, either.
Don't bother playing in a single deck blackjack game if it only pays out 6 to 5 for a blackjack, though. This is one of the worst rules variants in the game. It's more than enough to destroy whatever little gain you get from the smaller number of decks in the shoe.
In fact, this one change in the rules increases the house edge by a whopping 1.4%.
If enough people skip this version of the game, maybe the casinos will offer it less often.
This is a good example of how casinos take advantage of player ignorance. Everyone has learned that single deck games offer better odds, but few people understand the significance of that small change in the payout on a natural. The casino's banking on that, hoping that they'll attract naïve players. And for the most part, it works pretty well.
The house edge is the percentage of each bet that the casino expects to keep over a tremendously long period of time. So if we talk about a game with a 1% house edge, the casino expects to win $1 every time you bet $100. Of course, that NEVER happens in a single hand of blackjack. But over the course of thousands of hands, it can easily average out to that amount. In fact, the more hands you play, the closer you can expect to get to the expectation.
Eight decks have a higher house edge than six decks, and six decks have a higher house edge than two decks or one. Everything else being equal, go for the game with the fewer decks every time. The only exception is that 6:5 payout I already warned you about.
You might be playing in a single deck game with great rules and face a house edge of only 0.17%. The same game with the same rules might have a house edge of 0.46% if you double the number of decks in play. With four decks, the house edge jumps to 0.60%. With eight decks, you're looking at a house edge of 0.66%.
Counting cards is a popular advantage play technique where a player tracks the relative number of aces and tens in the deck compared to the lower cards. A deck with a proportionally higher number of high cards is more likely to result in a natural and the corresponding 3 to 2 payout.
This means you can raise your bets in certain situations and get a profitable situation.
But with more decks in play, it's hard to get into a positive expectation situation. Think about it this way:
You're playing in a single deck game. All the aces have already been dealt.
It's not impossible to get dealt a blackjack. It's probably a good idea to flat bet the table minimum at this point.
But in a shoe with 8 decks, you still 28 aces left in the deck. Your odds of getting a blackjack have still gone down, but they haven't dropped to 0.
And that works in reverse, too. The low cards are the ones which hurt your chances of getting a natural, so when they get dealt out, the deck improves. The amount that the deck improves is a function of how many decks are in play.
Card counters compensate for these additional decks by converting their count from a running count into a true count. They do that by dividing their count by the number of decks left in the shoe. This isn't the easiest math in the world, which is one reason counters love single deck games so much.
Fewer decks mean a great chance of getting dealt a natural. That 3 to 2 payout is the main reason that adding more decks hurts your odds. Change that payout to 6 to 5, and you've eliminated the edge that you gain by playing in a single deck game.
Having lots of decks in play means more to a card counter than most, although it hurts the odds for anyone playing in the game. It's hard to get an edge with that many cards in play, so the card counter gets fewer opportunities to raise his bets.
But even if you're just a basic strategy player, you should look for single deck games.