Most casino games are designed so the house always wins in the long run. Blackjack is the famous exception. Under the right conditions, a skilled card counter can flip the edge and actually expect to profit over time. It is the only widely available casino game where this is mathematically possible without cheating.

But card counting is widely misunderstood. Movies like 21 and Rain Man make it look like a feat of genius-level memorization. In reality, the core technique is surprisingly simple. The hard part is not the counting itself – it is doing it accurately under casino conditions while appearing to be a casual player.

What Is Card Counting?

Card counting is a strategy where you track the ratio of high cards to low cards remaining in the deck. You are not memorizing every card. You are keeping a running score that tells you whether the remaining shoe is favorable or unfavorable to the player.

Here is the key insight: in blackjack, the composition of the remaining deck changes the odds. Unlike roulette, where every spin is independent, each card dealt in blackjack removes that card from play. This means past results genuinely affect future probabilities.

When the remaining deck is rich in high cards (10s, face cards, and Aces), the player has an advantage. When it is rich in low cards, the dealer has an advantage.

Why High Cards Favor the Player

This might seem counterintuitive. High cards help both the player and the dealer, right? Not equally. Here is why a deck rich in high cards benefits the player more:

• Blackjack pays a bonus. A natural 21 pays 3:2 (or $15 on a $10 bet). Both the player and dealer get more blackjacks from a high-card-rich deck, but only the player gets the bonus payout. The dealer just wins even money.
• Doubling down is more profitable. When you double down on 10 or 11, you want to draw a high card. A deck full of 10s and face cards makes this much more likely.
• The dealer busts more often. Dealers must hit on 16 or less. With more high cards in the deck, the dealer is more likely to draw a card that pushes them over 21. Players can choose to stand on a low total, but the dealer cannot.
• Insurance becomes profitable. Normally insurance is a terrible bet, but when the deck is very rich in 10-value cards, the odds of the dealer having blackjack increase enough to make insurance mathematically correct.

Why Low Cards Favor the Dealer

When the remaining deck has a higher proportion of low cards (2 through 6):

• The dealer is less likely to bust when hitting on stiff hands (12 through 16)
• Players receive fewer blackjacks and their bonus payouts
• Doubling down on 10 or 11 is less effective because small cards do not help much
• The player’s strategic advantages shrink while the dealer’s fixed rules become less punishing

The Hi-Lo System: Card Counting Basics

The Hi-Lo system is the most popular card counting method. It was popularized by Edward Thorp in his 1962 book Beat the Dealer and later refined by Stanford Wong and others. It is a balanced, single-level count, which means it is relatively easy to learn.

How It Works

Every card that is dealt gets assigned a value:

As cards are dealt, you keep a running count by adding or subtracting based on each card you see.

Example: The first few cards dealt are 5, King, 3, 8, Ace, 6.

• 5 → +1 (running count: +1)
• King → -1 (running count: 0)
• 3 → +1 (running count: +1)
• 8 → 0 (running count: +1)
• Ace → -1 (running count: 0)
• 6 → +1 (running count: +1)

After these six cards, the running count is +1, meaning slightly more low cards have been dealt, and the remaining deck is marginally better for the player.

The True Count

In a multi-deck game (most casinos use 6 or 8 decks), the running count alone is not enough. A running count of +6 means very different things depending on whether there are 2 decks remaining or 5 decks remaining.

The true count adjusts for the number of decks left:

True Count = Running Count / Decks Remaining

If the running count is +6 and you estimate 3 decks remaining, the true count is +2. If the running count is +6 with only 1 deck remaining, the true count is +6 – a much more significant advantage.

Players estimate decks remaining by eyeballing the discard tray. This takes practice but does not need to be precise.

Using the Count to Bet

The true count determines two things: how much to bet and how to play.

Betting: When the true count is positive (deck favors the player), you increase your bet. When it is negative or zero, you bet the minimum. A common approach is the “1-12 spread” – bet 1 unit at a true count of 0 or less, and scale up to 12 units at high true counts.

Strategy deviations: At certain counts, the mathematically correct play changes from basic strategy. For example:

• At a true count of +3 or higher, you should take insurance (normally a bad bet)
• At a true count of 0 or below, you should hit 16 against a dealer 10 (normally a stand in some basic strategy charts)
• At a true count of +4, standing on 15 against a dealer 10 becomes correct

There are about 18 key deviations that capture most of the value. Memorizing all of them is unnecessary for a recreational counter.

The Math: Why Card Counting Works

The house edge in a typical 6-deck blackjack game with standard rules is about 0.5% for a basic strategy player. Card counting works by identifying moments when that edge shifts in the player’s favor.

Expected Value at Different Counts

Each +1 increase in the true count shifts the player’s edge by approximately 0.5%. Here is what that looks like:

A true count of +2 or higher means the player has a genuine mathematical edge. The counter bets big during these favorable moments and bets small (or sits out) when the count is negative.

Long-Term Profit

Even with a positive edge, the swings are enormous. A skilled counter using a 1-12 bet spread in a 6-deck game can expect to earn roughly 1 to 1.5 units per hour of play. With a $10 base unit, that is $10 to $15 per hour on average.

But “on average” hides brutal variance. Losing streaks of hundreds of hands are common even when the count is favorable. Professional counters need a bankroll of at least 200 to 300 maximum bets to weather the swings with a reasonable risk of ruin.

This is why card counting is not a get-rich-quick scheme. It is a grind that requires discipline, a substantial bankroll, and the ability to endure long losing stretches without deviating from the system.

Casino Countermeasures

Casinos are well aware that card counting works, and they have developed an arsenal of countermeasures over the decades.

Shuffle Frequency

The most common and effective countermeasure is frequent shuffling. Many casinos now use continuous shuffling machines (CSMs) that shuffle discards back into the shoe after every few hands. This makes card counting completely ineffective because the deck composition never becomes significantly skewed.

In shoe games without a CSM, casinos control the penetration – how deep into the shoe they deal before reshuffling. A casino that shuffles after dealing 4 out of 6 decks (67% penetration) gives counters far less opportunity than one that deals 5 out of 6 decks (83% penetration). Most casinos today offer 65-75% penetration, which limits but does not eliminate counting opportunities.

Detection and Backoffs

Casino surveillance teams use several methods to spot counters:

• Bet spread tracking: The most obvious tell is a player who bets $10 for twenty hands and then suddenly bets $200. Software systems track bet patterns and flag players with suspicious spreads.
• Play correlation: Casinos compare your playing decisions against basic strategy and known count-based deviations. If you take insurance only when the count is high, that is a strong signal.
• Behavioral cues: Experienced pit bosses watch for players who seem too focused on the cards, avoid conversation, or move their lips slightly while counting.

When a casino identifies a suspected counter, they have several options:

• Shuffling up: The dealer shuffles the shoe every time the player increases their bet. This eliminates any advantage without confrontation.
• Reducing penetration: Dealing fewer cards before reshuffling for that table.
• Backoff: A pit boss politely tells the player they are welcome to play any other game, but they can no longer play blackjack. This is legal in most jurisdictions.
• Trespass: In extreme cases, the casino bans the player from the property entirely.

Rule Changes

Some casinos have altered the rules specifically to combat counters:

• 6:5 blackjack payout instead of 3:2 (dramatically increases house edge for everyone)
• No mid-shoe entry (prevents counters from jumping in when the count is favorable)
• Restricting bet sizes for suspected counters while allowing normal play for others

Practical Takeaways

Card counting is legal. It is not cheating – you are simply using your brain and publicly available information. However, casinos are private businesses and can refuse service to anyone they choose.

Here are the realistic facts to keep in mind:

• The math works, but the edge is thin. Expect 0.5% to 1.5% advantage at best, with enormous variance.
• The Hi-Lo system is simple to learn but difficult to execute flawlessly in a loud, distracting casino environment for hours at a time.
• Bankroll requirements are steep. You need 200 or more maximum bets to have a reasonable chance of surviving the inevitable downswings.
• Casinos fight back. The countermeasures are sophisticated and effective. Most counters today supplement Hi-Lo with team play, disguise techniques, or advantage play in other games.
• Online blackjack cannot be counted. Virtual games shuffle after every hand, and live dealer online games use CSMs or shuffle very frequently.

Conclusion

Card counting is one of the most fascinating applications of probability in any game. It proves that blackjack is not purely a game of chance – the deck has a memory, and a player who tracks that memory can tilt the odds in their favor.

The Hi-Lo system is the foundation that most counters start with: assign +1 to low cards, -1 to high cards, convert to a true count, and bet accordingly. The math is sound, the edge is real, but the practical challenges – casino countermeasures, bankroll requirements, and psychological endurance – make it far harder than the movies suggest.

If you are drawn to the mathematical side of card games, poker offers a different kind of strategic depth. Unlike blackjack, where you play against the house and its fixed rules, poker pits you against other players where skill has an even larger impact. Tools like the AI Poker Tools Odds Calculator can help you build a sharper sense of probability and expected value – skills that translate across all card games.