What is card counting in blackjack?
Card counting is a technique where players track the ratio of high cards to low cards remaining in the shoe. When the deck is rich in high cards the player has a statistical edge, so counters increase their bet size in those situations and bet smaller when the deck favors the house.
How it works
The most widely known system is Hi-Lo. Each card dealt is assigned a value: 2-6 add +1 to the running count, 7-9 are neutral, and 10-A subtract 1. A high positive count means more high cards remain, which favors the player. Counters divide the running count by the number of decks remaining to get the ‘true count’ which guides bet sizing.
Is it allowed?
Card counting is not illegal but casinos actively discourage it. If a casino suspects you are counting they can ask you to stop playing blackjack, limit your bet spreads, or ban you from the property. Online blackjack using continuous shuffle machines is immune to counting since the deck is reshuffled after every hand. Live dealer online blackjack uses real decks but the penetration (how much of the shoe is dealt before reshuffling) is typically shallow, limiting any advantage.