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Last Updated: December 10, 2020

Why the Number of Decks Matter in Blackjack


One blackjack question I get from time to time is, “Why is the house edge less with fewer decks in blackjack for the basic strategy player?” Surprisingly, as far as I know, nobody has looked very deeply into it. I have most of the most advanced books on blackjack ever published and know some of the greatest mathematicians of the game personally.

In his book The Theory of Blackjack, author Peter Griffin touches on the subject in a couple pages. He compares a particular set of rules in both a single-deck and infinite-deck and notes the former has a house edge 0.69% less. Here is how he breaks down the reason:

  • Doubling Down: Almost half
  • Player blackjacks: 0.07%
  • Splitting: More than offsets the benefit of blackjacks
  • Standing on 12 to 16: “Presumably the remaining discrepancy.”
 

Griffin’s answer is a bit vague, but quite on target.

In an effort to shed more light on the topic, I did my own analysis. The rules I assumed were:

  • Dealer hits soft 17.
  • Blackjack pays 3 to 2.
  • Dealer peeks for blackjack with a ten or ace up.
  • Player may double on any two cards.
  • Player may not surrender.
  • Player may double after splitting.
  • Player may re-split any pair (including aces) up to three times.
  • Continuous shuffler used (cards shuffled after every hand).
  • Player uses basic strategy.
 

The way I did it was to start with a hypothetical balanced game, where the player played like the dealer and a mutual bust was a push. Then I added rule after rule, studying the effect on both a single- and eight-deck game, to get an idea of how each rule affects the house edge for both one and eight decks.

I won’t get into all the math here, but here is my executive summary of the reason fewer decks benefits the basic strategy player.

Player may stand on hard 12 to 16 76.9%
Player may double 48.5%
Player winning blackjack pays 3 to 2 10.8%
Dealer wins if player busts first -14.0%
Player may split -22.2%
 

For much more on my analysis, please visit Why the number of decks matters in blackjack.

I hate to beg, but this is one of my best articles in a long time so hope my readers find it.