Card counting is a blackjack technique for tracking the changing ratio of high cards (tens, face cards and aces) to low cards remaining in the shoe. When proportionally more high cards are left, the player gains a small statistical edge — naturals become more likely and the dealer busts more often — so the counter raises their bet; when low cards dominate, the edge swings to the house and the counter bets the minimum. It turns blackjack from a fixed-edge game into one where the player can occasionally hold the advantage.
The most common method is the Hi-Lo system. Each card is assigned a value: cards 2 through 6 count as +1, cards 7 through 9 as 0, and tens, face cards and aces as -1. The player keeps a running total as cards appear, then divides by the estimated decks remaining to get the "true count", which guides bet sizing.
Worked example: across several hands you have seen many low cards and your running count reaches +10. With roughly two decks left, the true count is 10 / 2 = +5 — a strongly ten-rich shoe that favours the player, so the counter increases the bet to exploit the temporary edge. The next shuffle resets the count to zero.
Why it matters: card counting is the rare advantage-play method that can give a disciplined player a genuine long-run edge over the casino.
Caveats: counting in your head is not illegal, but casinos are private operators and freely exclude or back off suspected counters, shuffle earlier, or restrict bet variation. It also demands perfect basic strategy, a real bankroll and practice; the edge only emerges over large volume.
Common mistake: assuming counting guarantees profit. It tilts a thin edge in your favour at best, easily nullified by detection or poor execution.
Compare to:
Basic Strategy, Shoe, Bust.