How Blackjack Works

Card Counting

Card counting is really just an extension of the basic strategy. It doesn't require a photographic memory or a degree in mathematics. Although the first card-counting systems were developed and published in statistical journals by mathematicians, the actual counting isn't that hard. The hard part is keeping from getting thrown out of the casino.

Counting cards is not cheating. The casinos tried to get laws passed that would make counting a crime, but the courts declared that counting is simply a skillful use of the information available to the player. Which means it's okay to count, right?


Not so fast. Casinos are private property. They can throw you off their property for any reason at all, including playing a game so well that they start to lose money. And once you've been kicked out, returning can result in a trespassing charge. We'll explain how to avoid getting caught in a minute, but first you need to learn how to count.

The basic strategy is based on odds that take into account all the cards in the deck. There's a slight flaw with that strategy, however: After a hand is played, the dealer puts the used cards in the discard tray, and deals the next hand with the remainder of that same deck. Approximately half to three-quarters of a deck might be used before the dealer reshuffles. That means that there are a lot of cards in the discard tray that a basic strategy is still accounting for. Card counting systems calculate the odds of a 10-value card being drawn based only on the cards still in the deck.

One common card-counting system assigns a value to certain cards in the deck:

  • Twos through Sixes are given a +1 value.
  • Tens through Aces are given a -1 value
  • Sevens, Eights, and Nines are valued at zero.

As the player sees the cards being played (and subsequently discarded), he adds those values together. From a starting point of zero, this "running count" fluctuates between negative and positive values. If the first hand dealt from a deck has a Two (+1), a Nine (0), a King (-1), an Ace (-1), a Ten (-1), and a Jack (-1), the running count is -3.

The higher the running count, the more low-value cards have gone into the discard tray. That means there is a higher percentage of high-value cards still in the deck. Why is that important? Recall how the basic strategy is based on the assumption that the next card will be a 10-value card. If you know that there is a greater percentage of 10-value cards in the deck than usual, that assumption -- and therefore the overall basic strategy -- becomes that much stronger.

So how does card counting change the rules of the basic strategy? It doesn't. What it does change is how much you bet.

A typical "system" player (someone who uses a card-counting system) will bet the table minimum when the deck is fresh. When the running count hits a certain level, such as +4 or higher, the player then makes a much larger bet or doubles down aggressively. The higher the count, the bigger the bet. The system player uses these beneficial odds to make a big win or two while the deck is "hot." If the count drops below zero or the deck is shuffled, he returns to the minimum bet.

In the next section, we'll explain how the casinos try to stop the counters and how the counters try to hide their advantage.