How card counting works in blackjack, and why casinos hate it

Card counting is a way of keeping a mental tally of which cards have already been dealt in blackjack, so a player knows when the remaining cards favour them. It works because blackjack, unlike roulette or slots, has memory: each card dealt changes the composition of what is left. It is not illegal, but casinos treat it as a threat and use a range of measures to neutralise it.

Why the cards left in the shoe matter

Blackjack is normally a game with a small house edge, even for a player who follows basic strategy, the mathematically optimal way to play each hand. That edge is not fixed from hand to hand, however. When the remaining shoe contains a high proportion of tens and aces, the player gains: blackjacks become more likely, and a natural pays 3 to 2 to the player but nothing extra to the dealer. Doubling down is more profitable, and the dealer, who must draw on 16 or less, busts more often.

When the shoe is rich in low cards, the opposite happens. A counter does not memorise individual cards. The aim is simply to know whether the remaining cards are richer or poorer in high cards than usual, and to bet more when the balance tips in the player's favour.

The Hi-Lo system explained

The most widely taught method is the Hi-Lo count, also called the complete point count. It was introduced by the mathematician Harvey Dubner in 1963 and later refined with Julian Braun and popularised by Edward Thorp. Each card is given a value, and the player keeps a single running total as cards appear.

• Cards 2, 3, 4, 5 and 6 count as +1, because removing low cards helps the player.
• Cards 7, 8 and 9 count as 0 and are ignored.
• Tens, face cards and aces count as -1, because removing them hurts the player.
• The count starts at zero after every shuffle; because the system is balanced, a full deck adds up to zero.

A higher positive count means more high cards remain. The Wizard of Odds guide to Hi-Lo sets out the full method, including how counters vary their play on certain hands depending on the count.

Running count versus true count

The running count is the raw total. On its own it can mislead, because most casinos deal from shoes of six or eight decks. A running count of +6 means much more when one deck remains than when five decks remain, since the surplus of high cards is spread across far fewer cards.

To correct for this, counters convert the running count into a true count by dividing it by the number of decks still to be dealt, estimated by looking at the discard tray. A running count of +6 with three decks left gives a true count of +2. A running count of +6 with six decks left gives a true count of +1. Bet sizes and strategy changes are keyed to the true count, not the running count.

Even with perfect execution, the advantage gained is small and swings in the short run are large. Many hours of play can pass with the count rarely rising high enough to matter, and a counter can lose heavily even when playing correctly.

From Thorp to the MIT team: a short history

The foundations were laid in 1956, when four US Army mathematicians at Aberdeen Proving Ground, Roger Baldwin, Wilbert Cantey, Herbert Maisel and James McDermott, published "The Optimum Strategy in Blackjack" in the Journal of the American Statistical Association. Working with desk calculators, they produced an early version of what became basic strategy.

Edward O. Thorp, a mathematician, used computer analysis to show that a player could gain an edge by tracking removed cards. His book Beat the Dealer, published in 1962, became a bestseller and is widely regarded as the birth of card counting. Its main system was a ten-count, and later editions adopted a point count based on Dubner's Hi-Lo.

The best-known counters are the MIT Blackjack Team, which operated from around 1979 into the 1990s. Groups of students and graduates used team play: spotters counted at several tables and signalled a big player to join when the count was high. Their story reached a mass audience through Ben Mezrich's 2003 book Bringing Down the House and the 2008 film 21, both of which took considerable liberties with events, as former team members told MIT's student newspaper The Tech.

Is card counting legal, and why can casinos bar you?

Counting cards in your head is not a crime under federal law or the law of any US state. Using a device is a different matter. In Nevada, NRS 465.075 makes it unlawful to use or possess with intent to use any device or software designed to track cards or analyse probabilities in a licensed casino, and violations are treated as felonies.

Legality does not mean a right to play. In Nevada and most other jurisdictions, casinos are private businesses that may refuse blackjack action to players they suspect of counting, a practice known as being backed off or barred. New Jersey is the main exception. In Uston v. Resorts International, decided in 1982, the New Jersey Supreme Court ruled that Atlantic City casinos could not exclude the famous counter Ken Uston, because the state Casino Control Commission held exclusive power over the rules of the games. Casinos there instead rely on countermeasures.

How casinos fight back

• Continuous shuffling machines feed discards straight back into the shoe, so the count never builds and counting becomes useless.
• Shallow penetration: dealers place the cut card early and reshuffle with many cards undealt, before the count can become meaningful.
• Betting limits and surveillance staff trained to spot players who raise their bets sharply when the count rises.
• Reduced payouts such as 6 to 5 for a blackjack instead of 3 to 2, which increases the house edge enough to wipe out much of a counter's advantage.
• Shuffling at will, flat-betting restrictions and simply asking a suspected counter to play other games.

Online blackjack offers no opening either: digital games typically reshuffle the virtual deck after every hand. For most players, card counting is best understood as a piece of gambling history and a lesson in probability rather than a practical way to make money. Blackjack remains a game the house is built to win over time.