Setting the context
Blackjack is a game of chance with a built-in casino advantage, and the number of decks and the way cards are shuffled change that advantage only slightly. No strategy, system, pattern or timing changes the odds in the player’s favor over time, and most players lose money over time. Nobody should gamble money they cannot afford to lose, and local support services can help anyone for whom gambling has become a problem.
How many decks are used
A standard deck has 52 cards. Blackjack is dealt from different quantities:
| Format | Cards |
|---|---|
| Single deck | 52 |
| Double deck | 104 |
| Six decks | 312 |
| Eight decks | 416 |
Single-deck and double-deck games are usually pitched by the dealer, who holds the cards. Larger counts are generally dealt from a shoe, a box that holds the cards and lets the dealer slide them out one at a time. Multi-deck shoes also make it easier to deal to many players and to keep the pace brisk.
What deck count changes
With more decks, the composition of the cards changes a little less after each card is dealt, and the probability of key events moves slightly. One simple example is a natural. With a single deck, the chance of being dealt an ace and a ten-value card as the first two cards is 2 × (4/52) × (16/51) = 128/2,652, about 4.83 percent. With an unlimited number of decks the chance is 2 × (1/13) × (4/13) = 8/169, about 4.73 percent. The difference looks tiny, but it is real, and it is one of several small effects that add up.
The commonly cited result is that, with other rules equal, moving from a single deck to a large shoe adds roughly half a percentage point to the casino’s edge. That is not a large number, but in blackjack, where the whole edge is often under one percent, it is substantial in relative terms.
The important qualifier is “other rules equal”. Deck count is only one of the posted rules. A single-deck game that pays 6 to 5 for naturals can cost the player more than a multi-deck game that pays 3 to 2, because the reduced payout adds more to the edge than the lower deck count takes away. Other rules, such as whether the dealer hits soft 17, which hands can be doubled and whether surrender exists, can matter as much. The way to compare tables is by reading the whole set of rules rather than judging on deck count alone.
What shuffling does
A shuffle restores the cards to a random order, and the game restarts with the full composition. Between shuffles the cards that have been dealt are removed from the pool, which is why the cards left are not identical to a full deck.
Casinos choose how deep to deal before shuffling. Dealing fewer cards before the cut means the remaining composition rarely strays far from a fresh deck. Dealing deeper gives slightly more variation. This is one of the controls casinos use, and it is related to the reasoning behind why venues can refuse play from card counters.
Types of shuffling machines
Machines have become common for several reasons: they speed up the game, reduce wear and handling by dealers, and reduce the opportunity for certain kinds of cheating. There are two broad types.
Batch shufflers mix one set of cards while another is in play, so there is little pause between shoes. The game still involves a depleting set of cards, and the rest of the rules are unchanged.
Continuous shuffling machines take the cards the dealer has used and feed them back into the mix, so the machine is constantly reshuffling. Effectively every hand is dealt from a nearly full set. For a player, it means the history of earlier hands carries no useful information. The per-hand edge is not very different from a standard multi-deck game, but the effect on speed is significant: because the dealer never pauses to reshuffle, more hands are played per hour.
That last point is easy to miss. If the edge per hand is unchanged but the number of hands per hour rises, then the average loss per hour rises with it. As a hypothetical example, a $10 bettor facing a 0.5 percent edge who plays 60 hands an hour wagers $600, an expected loss of $3 per hour. At 80 hands an hour the wager is $800 and the expected loss is $4. Neither figure predicts a particular evening, but the second is a third larger. This is the same arithmetic that sits behind hourly expected loss at a roulette table.
Random order, no memory
Whatever the method, a fair shuffle makes the order of the cards unpredictable. A run of high cards in one round gives no useful guide to the next one, and the belief that “low cards are due” is another form of the gamblers fallacy. The exception is the narrow case of cards actually removed from a depleting shoe, which affects probabilities only slightly and is not something a casual player can use.
Online and live formats
Software-based blackjack does not shuffle physical cards. Random number generators are used to produce a random order, and many online games draw each hand from a freshly randomized set, so deck count may exist only as a rule setting. Live dealer games use real cards at a table streamed to the player, often with shuffling machines, and have a similar deck count and shuffle method to a land-based table. Online versus land-based, the key point is the same: the posted rules, not the surroundings, determine the cost.
What to take from this
- More decks tend to give a slightly higher casino edge, with other rules equal.
- Fewer decks do not guarantee a better game if other rules, particularly the natural payout, are worse.
- Shuffling machines change the pace more than the per-hand edge, and a faster pace raises the expected loss per hour.
- No deck count or shuffling method makes the game favorable to the player over time.