Most people know that you should play fewer blackjack games if the rules for both are the same. If everything else is equal, the rule of thumb is that more decks mean a greater advantage for the house.

You can see the difference in house edge between a blackjack game that has eight decks and one with the same rules. If you want to keep your house edge below 1%, 0.6% can make a big difference.

Even an eight-deck game has a 0.3% advantage over a two-deck one. A casino will often have additional rules to go along with the deck differences to bring the house edge for their game closer.

## House Edge and Basic Strategy are affected by the number of decks.

Although it might not seem that the number of decks makes a big difference, it could be. What’s the difference between a deck that has four of 52 cards that have aces and one that has 32 of 416 cards that have aces? There is still a 1/1 chance of getting an Ace.

This is because if you take a card out of a deck in a single-deck game, it will have a greater effect than if it was in a multi-deck game.

If you remove an ace from one deck, the probability of getting an Ace is now 3/51 or 1/17.

If you take the ace from an eight-deck pack, you’ll still have 31 cards of 415 that are aces. About 1 in 13.8 people have a chance of getting an Ace.

These are other considerations when considering basic Blackjack Strategy in single-deck versus multiple-deck blackjack games.

## Multi-deck play will result in a natural less often

Blackjack is big on the natural. In fact, the game’s name is after the other word for natural. Blackjack pays at three to two odds. This is why blackjack is so mathematically rewarding.

A single-deck game will see a natural approximately once in every 20 or 21 hands. How did we arrive at that number?

There is a 1/113 chance that you will get an ace with your first card. To make the blackjack, you will need a 10, so you can use a 10. There are 16 cards that are worth 10 in a 51-card deck. This gives you a chance of getting a 10, of which there is a 16/51 probability.

You can get an ace and a 10, but only if you multiply these probabilities. In other words, multiply them. This would mean that the probability of a given event is 1/13 X (16/51), or 16/663.

However, this is not the only way you can get a blackjack. A 10 could be your first card and an Ace as your second, so that you can multiply the chance of getting an Ace and a 10, and the probability of getting a 10.

While the chance of getting an Ace on your first card is still 1/13 (or a 10 on your second), it is now 128/511.

1/13 X 128/511 = 1/28/6643

To double the chance of you getting the hands in the opposite order, multiply that by 256/6643. This is roughly the same probability as getting a blackjack at 1 in 26.

If percentages are more your thing, the probability of winning a blackjack in a single-deck game is about 4.8% (in single deck) and 3.8% (in eight-deck).

## Multi-deck Blackjacks will give the dealer more chances to win a blackjack

You should also consider the possibility that the dealer will get a blackjack. You can get a push if you and the dealer get a blackjack.

A single-deck game will see the dealer get an ace as her initial card 3/5 times. (Notice how numbers change to reflect cards that have been dealt. She will get a 10 as her 2nd card 15/49 times.

Now you have 3/50 x 45/49. To account for the possibility of getting the blackjack in an opposite order, multiply that number by two and you will get approximately 90/2450. This is about 1 in 27.2 or 3.67%.

Remember what we have already discussed in this post, and what we showed in the last section. The effects of cards removed from a deck are less effective when there are more decks. These fractions’ denominators don’t change much.

The probability that the dealer will also get a blackjack and end up with a shove is about 1 in 22 instead of once out of 27 hands. You’ve already seen enough examples to know how I calculated it.

## Doubling down doesn’t work as well with multiple decks

These aren’t the only differences in multi-deck games. Another example:

Let’s say your first two cards were a 9 or 2 and you have a total of 11. The dealer has a 6 showing. Anybody who is familiar with basic strategy will know that double down is the best move.

You want to score a 10, which will give your total of 21.

Out of 513 cards, there are 144 cards worth 10, which is the minimum amount of cards you can have in an eight-deck game. This is a 28.07% chance. This is a big difference.

It’s not that there aren’t more 10s in this deck. There are more cards that are worth 9, 2, or 6 in the deck.

Also Read: Your First Game of Blackjack in a Casino

## Standing on a stiff hand doesn’t work as well with multiple decks

A stiff hand has a total of 12-16. These hands are not likely to win unimproved but are also more likely to bust if they take another card. If your dealer’s face up card is low (6 or less), you will stand more often and hope that the dealer will bust. You’ll hit if the dealer’s face up card is higher (7 or more).

When the dealer holds a low card, standing is more effective than when she has a high card. The dealer’s chance of having a 10 in the hole makes it easier for her to stand. Because there are more cards in a deck, the chance of her getting a smaller card from a multiple deck is higher.

Because the house rules require that the dealer hit more frequently than the player, the casino benefits from a greater number of smaller cards.

## In a multiple deck game, counting cards doesn’t work as well.

Counting cards works by tracking the percentage of 10s and aces in the deck relative to the smaller cards. If you have seen four aces in a single deck game, the chances of you getting a blackjack are now 0.

However, the chance of getting a Blackjack in an eight-deck game is lower if you see four aces being dealt. But it’s still a lot better than 0. You start with 32 cards in an eight-deck deck game. After seeing the four dealt aces, you have 28 cards left.

This is why skilled card counters often convert their running count to a true count. This is done by taking the remaining decks and multiplying the running count by this number.

A count of +1 will make you take different decisions than a count with +4. You won’t bet as often. It also has fundamental strategy implications.

## Conclusion

You should now be able to understand the reason basic strategy changes depending on how many decks are in play. This is why the house edge increases when there are more decks of cards in the game.

This boils down to getting less blackjack with more tie when you do win a blackjack. You won’t get as much boost from doubling down or the dealer busting less often if you have a strong hand.

For the inspiration and analysis in this post, I am grateful to the America Mensa Guide for Casino Gambling. You can buy a second-hand copy at any number websites if you don’t have it yet. Although it is out of date in many ways, the book was written over 20 years ago. However, I was able to learn how to calculate gambling and casino math through the explanations provided by that book.