Card Counting

Sounds like something out of a Hollywood movie, but it’s actually just simple math.

In blackjack, every card that gets dealt changes the odds of the next hand. Unlike roulette or slot machines, past cards matter because they affect which cards are still left in the deck.

The most common system is called Hi-Lo. Low cards (2–6) are counted as +1, high cards (10s, face cards, and Aces) as -1, and everything else as 0. As the game progresses, players keep a running total. A high positive count means there are more high-value cards remaining, which slightly increases the player’s advantage.

The math is surprisingly small. In multi-deck games, players divide the running count by the number of decks left to get the true count, which tells them how favorable the deck really is. When the count is high, they increase their bets. When it’s low, they bet the minimum. That’s the entire strategy.

The most famous example is the MIT Blackjack Team, a group of students who used mathematics, teamwork, and disciplined betting to win millions from casinos during the 1980s and 1990s. They weren’t cheating — they were simply making better decisions based on probability.

Card counting is legal in many places because players only use information visible on the table. Casinos, however, don’t like losing their edge, so they often ask suspected card counters to leave. In the end, card counting isn’t about having a photographic memory — it’s about using a small mathematical advantage consistently over thousands of hands.