Card counting in blackjack

A trait shared by every gambling game is unpredictability: you can’t guess the next card for certain. But in blackjack you can at least gauge the likelihood of different outcomes. Card counting doesn’t guarantee a win — it only lets a player judge which cards might still come out of the deck, based on what’s already gone to the discard tray. Below we break down how it really works: from the simple Hi-Lo system to true count, deck penetration, alternative systems and a well-judged bet spread.

What card counting in blackjack is and why it matters

Blackjack’s rules make the big cards — tens, Jacks, Queens, Kings and Aces — very valuable: they make it easier to build a blackjack or at least a strong hand worth 20. Small cards, by contrast, tend to lead either to a weak hand or to a bust¹. If you count the nines among the big cards too, then at the start, with a full deck, big and small cards are roughly even. That balance is disturbed as particular cards come out and land in the discard tray.

Card counting is exactly about understanding what proportion of big and small cards is left in the shoe (the remainder of the deck). Working from that, a counter raises their bet when the chances of winning are higher, and lowers it when a loss is likely. Counting on its own doesn’t guarantee success — it only helps you manage your bankroll more wisely. Even used perfectly, it adds an edge of just 1–2% over play without counting, and only over certain stretches of a session, while remaining hard work. Even so, dedicated players will try anything to improve their chances or at least stretch their bankroll for as long a game as possible.

A little history: Edward Thorp and the MIT team

One of the pioneers of the mathematical approach to blackjack is Edward Thorp — an American maths professor who worked at several leading universities. He became known to the wider public through his bestseller Beat the Dealer (1962), where he described, in mathematical terms, how to gain an edge in blackjack by analysing the remaining shoe.

His ideas were developed by the so-called MIT (Massachusetts Institute of Technology) team — a handful of gifted mathematicians who worked out the probabilities in various gambling games. Through the 1960s and 70s their work became a genuine headache for casinos. Since then, it has to be said, the gambling houses have gone to great lengths to neutralise the edge of a mathematically savvy player.

The Hi-Lo system: the basics

Blackjack moves fast, and literally memorising every card that’s gone to the discard tray is impossible — and it isn’t necessary either. So the Hi-Lo (high and low cards) system offers a stripped-down counting model, widely used thanks to how easy it is to apply while still being reasonably effective. If you’re just getting into the subject, it makes sense to start here, and it’s handy to keep the basic terms nearby in the glossary.

Card values

While the deck is full (all cards in their original proportion), the sum of the values is zero. From there, the count runs according to which cards go to the discard tray: for some we add points, for others we subtract.

With a high positive balance, the chance of a blackjack or simply a strong hand rises — so you can raise your bet. And vice versa: if the count is negative, it’s not worth risking large sums, and sometimes it even makes sense to leave the table.

Running count

Counting every card that goes to the discard tray by the system just described is called the running count. On its own it doesn’t give a clear picture of the balance of cards left in the deck — unless you’re playing with a single deck. Suppose 12 small cards have left play with no middle or high cards at all — formally that’s a huge advantage for the player. But there’s a catch: it’s an impressively good situation with one or two decks, whereas blackjack is also played with eight. If there really are eight decks, the balance has shifted, but not so dramatically — plenty of small cards still remain in the shoe.

True count

To make the counting result more relevant, players use the true count: for this, you divide the running count by the number of decks still in play. Say the running count is +8. For a single-deck game that is the true count — an excellent surplus of high cards in the shoe, good for the player. But in an eight-deck game the true count is +8 / 8 = +1, which is far less impressive.

Converting to a true count takes only basic arithmetic, yet it shows the real picture without distortion. The more decks are still in play, the more important it is to go by the true count rather than the running count. And count not by the starting number of decks, but by their approximate number in the shoe at the current moment.

Important: divide the running count by the number of decks still left in the shoe, not the original number. Otherwise, by the end of the shoe the true count will be systematically understated, and you’ll miss exactly the hands the whole thing was for.

How the count becomes an edge

A true count means more than just an abstract sense that the game is favourable — behind it sits a calculated mathematical probability of how the count affects your edge. At a high true count the casino’s edge is fully neutralised, and it now sits with the customer. The figures in the table below have been verified against the mathematical simulations described in mathematician Don Schlesinger’s Blackjack Attack (1997).

That said, the frequency reality (the grind — the distribution of high and low true counts) is that roughly 60% of all playing time the true count is zero or below (the game is potentially unprofitable for the customer), and only about 25% of the time does it reach or exceed +2 (maximum value). In other words, for most of a session the edge is still with the house, and all the counter’s profit is gathered in those rare stretches when the count swings sharply into the plus.

Deck penetration: the crucial factor

You never play a shoe all the way to the end: when relatively few cards are left, the dealer mixes the discards back in and reshuffles the whole deck, effectively starting from a clean slate. Deck penetration measures the point at which the dealer mixes the discards in: if it’s 50%, the reshuffle happens once half the deck has gone to the discards.

The higher the deck penetration, the better for the player: only when relatively few cards remain in the shoe does the true count start to give a real picture of the odds. Experts reckon that at 50% deck penetration counting is still practically useless, especially with many decks, and it only starts to matter from 70–75%.

Casinos know players can count cards, and they’re not keen on it. There are at least three ways to hinder a counter:

1. shuffle the cards more often — especially if the player’s behaviour points to active counting;
2. use a shuffle machine — the dealer can add the discards to it after literally every hand, driving deck penetration practically to zero;
3. move the counter away from the table — counting itself isn’t banned, but neither is this behaviour by the casino.

Important: counting cards without accounting for deck penetration is pointless. Before you sit down, assess how deep the dealer places the cut card — with shallow penetration, even a perfect count gives no edge.

Other counting systems

Hi-Lo is the most popular because it’s both relatively simple and accurate enough. Still, it would be unfair to ignore the other systems — each has its strengths.

• KO (Knock Out) — the same as Hi-Lo, but a seven that goes to the discards counts as +1 rather than 0, which is why the system is considered unbalanced². Overall it’s very simple and suits beginners, and it’s prized for removing the need to convert the running count into a true count — that’s the whole point of the built-in imbalance. That said, many experienced players consider Hi-Lo a touch more accurate.
• Red 7 — something between Hi-Lo and KO: a red seven counts as +1 and a black one as 0. At first glance that seems illogical, but in essence the system lets you skip converting to a true count (one operation fewer) while describing the edge more accurately than the rather crude KO. Tracking sevens by colour makes the count slightly harder, but the system’s author, Arnold Snyder, substantially improved its accuracy over KO.
• Hi-Opt II — a very complex system: 2, 3, 6 and 7 are +1; 4 and 5 are +2; 8, 9 and the Ace are 0; and all ten-value cards are −2. It’s considered very effective and is popular with professionals, including for play that doesn’t strictly follow basic strategy, but it demands enormous focus — any slip badly breaks the maths. Although the balance doesn’t change when an Ace comes out, specialists advise counting the Aces separately; the true count has to be kept separately here too.
• Omega II — similar to Hi-Opt II, but the six is +2 rather than +1, and the nine is −1 rather than 0. One of the most accurate systems, but it involves complex counting, a parallel Ace count and a mandatory true count. Professionals love it, while beginners will have to train hard to get the most out of it.

A beginner shouldn’t even attempt the high-difficulty systems. But within your own level you can pick any of them — the only thing that matters is not to flit between them until you’ve mastered one reliably enough.

Bet spread and bankroll management

Counting doesn’t guarantee wins — it only tells you when the odds are higher (bet bigger then) and when they’re lower (cut the amount). In essence, sensible bet management — the bet spread — is itself a part of the card-counting strategy³.

Even while counting and even while banking on an edge over the casino, a player has to understand: that edge is minimal. You might not even notice it with the naked eye — formally you still lose about half the time, and losses, like wins, sometimes stack into long streaks. Those streaks create the risk of ruin: you can blow your whole bankroll before the average catches up with your edge — all the more so since the house is ahead for most of a session.

Important: always take a bankroll with enough of a buffer to absorb several losses in a row — including doubles, splits and re-splits. Without that buffer you risk being knocked out before your edge has a chance to work.

Does counting work online

Venues have no interest in a player gaining an edge through counting, so they’ve developed measures against Hi-Lo and other systems. The most impenetrable is the random number generator (RNG): at these virtual tables there’s no discard tray at all — every hand starts with a full set of cards, so there’s simply nothing to count.

A live dealer in an online casino often works on a similar principle: they have a CSM (continuous shuffling machine) — an automatic device for quick shuffling, into which the discards can be added after almost every hand. That sharply cuts deck penetration and also makes counting pointless. Some live dealers don’t use a CSM, but even then remember the plastic cut card: the dealer places it in the middle in advance and, on reaching it, immediately mixes the shoe with the discards, preventing too deep a penetration.

Legality and reality

No country’s laws ban card counting, and a player is unlikely to find a rule against counting at any particular land-based venue. But in practice casinos don’t want you to count successfully, since that neutralises their edge. It’s fine when a venue fights counting with neutral methods — shuffling the cards more often, say. Some managements act less pleasantly: you’ll be firmly asked to move to another table, breaking your count and sometimes denying you access to your favourite version of the game.

In an online casino it’s far easier to count — no one can see you there. But the most advanced venues analyse behavioural signals and still work out a counter, moving them to another table on one pretext or another. Even so, they usually skip the crude methods here: on suspecting a counter, they simply feed a command into the dealer’s earpiece to reshuffle the cards at once.

Practice

Even a seemingly simple system is unlikely to click for a beginner on the first try — you have to practise first. To get results, go step by step:

1. start with one simple system, such as Hi-Lo;
2. train at home first, even with an ordinary deck of cards;
3. to pick up the pace and mimic a real game, use a trainer — there’s one on our site too;
4. only once you’ve fully mastered one system, move on to the next;
5. once you’ve got the simple systems down confidently, try the complex ones built for professionals.

And remember: card counting doesn’t guarantee wins. It only gives a small edge over the casino — and only on condition that you not only count correctly but also apply the bet spread correctly. From there it’s worth sharpening your basic strategy: without flawless play by the chart, no amount of counting will save you, but together they make up the player’s full arsenal.