Card counting is a blackjack strategy used to determine whether the player or the dealer has an advantage on the next hand. Card counters are advantage players who try to overcome the casino house edge by keeping a running count of high and low valued cards dealt. They generally bet more when they have an advantage and less when the dealer has an advantage. They also change playing decisions based on the composition of the deck.
Basics
Card counting is based on statistical evidence that high cards (aces, 10s, and 9s) benefit the player, while low cards, (2s, 3s, 4s, 5s, 6s, and 7s) benefit the dealer. High cards benefit the player in the following ways:
They increase the player's probability of hitting a Blackjack, which often pays out at 3 to 2 odds (although some Casinos pay at 6 to 5). Doubling down increases expected value. The elevated ratio of tens and aces improves the probability that doubling down will succeed. The most common hand values that the player doubles down on are 11, 10, and 9; and drawing a high card to these will make a strong hand. They provide additional splitting opportunities for the player. They can make the insurance bet profitable by increasing the probability of dealer blackjack. They also increase the probability the dealer will bust, in the event that the dealer shows a low up-card (i.e. 2-6). This also increases the odds of the player busting, but the player can choose to stand on lower totals based on the count.
On the other hand, low cards benefit the dealer. The rules require the dealer to hit stiff hands (12–16 total), and low cards are less likely to bust these totals. A dealer holding a stiff hand will bust if the next card is a 10.
Card counters do not need unusual mental abilities; they do not track or memorize specific cards. Instead, card counters assign a point score to each card that estimates the value of that card. They track the sum of these values with a running count. The myth that counters track every card was portrayed in the 1988 film Rain Man, in which the savant character Raymond Babbitt counts through six decks with ease, and a casino employee comments that it is impossible to do so.
Systems
Basic card counting systems assign a positive, negative, or zero value to each card. When a card is dealt with, the count adjusts by that card's counting value. Low cards increase the count; they increase the percentage of high cards in the deck. High cards decrease the count for the opposite reason. For example, the Hi-Lo system subtracts one for each 10, jack, queen, king, or ace and adds one for any card between 2 and 6. 7s, 8s, and 9s count as zero and do not affect the count.
A card counting system aims to assign point values roughly correlating to a card's effect of removal (EOR). The EOR is the estimated effect of removing a given card from play. Counters gauge the effect of removal for all cards dealt and how that affects the current house edge. Larger ratios between point values create better correlations to actual EOR, increasing the efficiency of a system. Such systems are classified as level 1, level 2, level 3, and so on. The level corresponds to the ratio between values.
The Hi-Lo system is a level-1 count; the running count never increases or decreases by more than one. A multilevel count, such as Zen Count, Wong Halves, or Hi-Opt II, further distinguishes card values to increase accuracy. An advanced count includes values such as +2 and −2, or +0.5 and -0.5. Advanced players might also keep a side count of specific cards like aces. This is done where betting accuracy differs from playing accuracy.
Many side count techniques exist, including special-purpose counts used for games with nonstandard profitable-play options such as an over/under side bet.
Keeping track of more data with higher level counts can hurt speed and accuracy. Some counters earn more money playing a simple count quickly than by playing a complex count slowly.
This table illustrates some example counting systems.
Design and selection of systems
The primary goal of a card counting system is to assign point values to each card that roughly correlate to the card's "effect of removal" or EOR (that is, the effect a single card has on the house advantage once removed from play), thus enabling the player to gauge the house advantage based on the composition of cards still to be dealt. Larger ratios between point values can better correlate to actual EOR, but add complexity to the system. Counting systems may be referred to as "level 1", "level 2", etc., corresponding to the number of different point values the system calls for.