Card counting systems in blackjack are a means of advantage play in which you gauge how favorable or unfavorable the deck is, and then betting more or less based on that estimate. This estimate is arrived at, not by memorization, but by a heuristic system which assigns a point value to various cards in the deck. Once a card has been removed from the deck, it either makes the deck more favorable for the player or less favorable for the player. This effect is tracked by adding and subtracting numbers from a mental tally called “the running count”.

Imagine that you’re playing roulette, and you place a bet on red, and you want to know what your chances of winning are. The calculations are simple enough; you simply divide the number of red slots on the wheel by the total number of slots on the wheel. On an American roulette wheel, you’d have 18 red slots, 18 black slots, and 2 green slots, for a total of 38 slots on the wheel. 18 divided by 38 is 47.37%, which is your chance of winning. No matter whether you win or lose on that bet on red, if you bet red again on the next spin of the wheel, your odds of winning are the same: 47.37%. They don’t change based on what’s happened before.

Now suppose you’re playing a card game similar to roulette, where the dealer turns over a card, and you bet on whether or not the next card is going to be red or black. The deck of cards has 18 red cards in it, 18 black cards in it, and 2 jokers. If you bet on red, you have a 47.37% chance of getting a red card. Now suppose that card is discarded, and you’re dealt another card. Have the odds of getting another red card changed? The answer should be obvious, because now there are only 37 cards total in the deck, and only 17 of them are red, so the odds of winning become 17/37, which equates to 45.95%. The odds of getting a black result have increased to 18/37, or 48.65%.

Now suppose you get red five times in a row. If you’re playing roulette, then your odds of winning your next bet on red is still 47.37%–they don’t remove red slots on the wheel when you’ve won. But in this imagined card game we’ve come up with, the difference is significant after five red wins in a row. The chances of being dealt a red card on the next play have gone down to 13 divided by 33, or 39.4%. Meanwhile, the chances of being dealt a black card on the next play have gone up to 18/33, or 54.54%.

Now suppose you only bet a single dollar on the first five bets, so you’ve lost $5. But now that your odds of winning (by betting on black) are more likely than not, you raise the size of your bet to $20. Your expected value is 54.54% of that $20, or $10.90. And if you kept playing long enough, you’d continue to find opportunities to make bets with a positive expectation.

That’s a remarkably accurate description of how card counting systems work, but instead of tracking the colors of the cards, you’re tracking whether the cards are high or low. In some systems, what constitutes “high” or “low” varies based on goals having to do with accuracy and ease of play. Either way, you’ll know when the deck is in your favor, because when the deck has lots of high cards in it, you’re more likely to get a 3/2 payout on a natural 21, and you can bet more to take advantage of the situation. You’ll bet less when the deck has lots of low cards in it, because the dealer is less likely to bust, which isn’t a favorable situation for the player.

Some of the most commonly used counting systems include the following:

- The 10 Count
- The Hi Opt 1 Count
- The KO Count
- The Omega 2 Count
- The Red Seven Count
- The Revere Advanced Point Count
- The Unbalanced Zen Count
- The Uston Advanced Count
- The Zen Count
- The Wong Halves Count

Each card has a value to a blackjack counter, and that value is called “the effect of removal”, or “EOR”. The EOR for the aces and fives have the largest effect on a player’s chances. If you remove all the aces from the deck, then a natural 21 is impossible, which subtracts considerably from the player’s chances of getting ahead. On the other hand, if you remove all of the fives from a deck, the EOR is so great that it would make almost any blackjack game a positive expectation game for the player.

The simplest card counting system is called an “Ace-Five Count”, and you simply add 1 to the count every time a five is dealt and subtract 1 from the count every time an ace is dealt. The higher the count is, the more you bet. If you have a wide enough betting range, say anywhere from 1 unit to 10 units, this is a remarkably easy and effective system. Most people are looking for more of an edge than this simple count offers, though, and they use slightly more complicated systems. (Something to notice about this system is that there are just as many +1 cards in the deck as there are -1 cards in the deck—this is called a “balanced” counting system, as a result. Not all systems are balanced.)

All card counting systems estimate multiple factors related to their effectiveness. The first of these is called “betting correlation”, and it’s probably the most important one. This number determines how well the count estimates the player’s edge—this is important for making decisions about betting amounts. The second of these is called “playing efficiency”, and this estimates how well the count takes into account changes to basic strategy. This factor isn’t as important as it may seem, but players who want to squeeze every percentage point of value out of their blackjack card counting system pay attention to it. The third and possibly least important factor is the “insurance correlation”, which is an estimate of how well the system predicts whether or not insurance is a good bet.

We already discussed balanced systems versus unbalanced systems, but just to reiterate, a balanced system is one in which the count evens out to 0 when you count through the entire deck. For example, the Hi-Lo system, which is quite popular, is a balanced system that counts aces and tens as -1 and counts any card ranked 2 through 6 as +1. There are 20 cards in each category, positive or negative, and when you finish counting through a deck, you’ll have a total running count of 0. In an unbalanced system, this isn’t true.

The Hi-Lo system is also an example of a single level system, because the cards are all valued at either +1 or -1. A higher level system might give different values to different cards. For example, if a system counts aces as -2 and fives as +2, and then counts tens as -1 and 2s, 4s, and 6s as +1, then the system would be a multi level technique.

Some card counting systems go even further and require you to keep a side count of the aces that have been dealt. The more complicated the rules for the system are, then the more accurate it is, but you need to remember that your system should also be practical. You’ll be counting cards in a crowded and noisy casino, so you need to be able to concentrate without looking like your concentrating. Practice is necessary, but so is a simple enough strategy.