One of the most interesting aspects of blackjack is the
probability math involved. It’s more complicated than other
games. In fact, it’s easier for computer programs to calculate
blackjack probability by running billions of simulated hands
than it is to calculate the massive number of possible outcomes.
This page takes a look at how blackjack probability works. It
also includes sections on the odds in various blackjack
situations you might encounter.
An Introduction to Probability
Probability is the branch of mathematics that deals with the
likelihood of events. When a meteorologist estimates a 50%
chance of rain on Tuesday, there’s more than meteorology at
work. There’s also math.
In a single deck blackjack game - if you're not counting cards - the probability that the next card will be a 10/J/Q/K is 16/52. I'm trying to figure out how to adjust the probabilities when you are counting cards.
Probability is also the branch of math that governs gambling.
After all, what is gambling besides placing bets on various
events? When you can analyze the payoff of the bet in relation
to the odds of winning, you can determine whether or not a bet
is a long term winner or loser.
The Probability Formula
The basic formula for probability is simple. You divide the
number of ways something can happen by the total possible number
of events.
Here are three examples.
Example 1:You want to determine the probability of getting heads when
you flip a coin. You only have one way of getting heads, but
there are two possible outcomes—heads or tails. So the
probability of getting heads is 1/2.
You want to determine the probability of rolling a 6 on a
standard die. You have one possible way of rolling a six, but
there are six possible results. Your probability of rolling a
six is 1/6.
You want to determine the probability of drawing the ace of
spades out of a deck of cards. There’s only one ace of spades in
a deck of cards, but there are 52 cards total. Your probability
of drawing the ace of spades is 1/52.
A probability is always a number between 0 and 1. An event
with a probability of 0 will never happen. An event with a
probability of 1 will always happen.
Here are three more examples.
Example 4:You want to know the probability of rolling a seven on a
single die. There is no seven, so there are zero ways for this
to happen out of six possible results. 0/6 = 0.
You want to know the probability of drawing a joker out of a
deck of cards with no joker in it. There are zero jokers and 52
possible cards to draw. 0/52 = 0.
Probability Of Blackjack With One Deck
You have a two headed coin. Your probability of getting heads
is 100%. You have two possible outcomes, and both of them are
heads, which is 2/2 = 1.
A fraction is just one way of expressing a probability,
though. You can also express fractions as a decimal or a
percentage. So 1/2 is the same as 0.5 and 50%.
You probably remember how to convert a fraction into a
decimal or a percentage from junior high school math, though.
Expressing a Probability in Odds Format
The more interesting and useful way to express probability is
in odds format. When you’re expressing a probability as odds,
you compare the number of ways it can’t happen with the number
of ways it can happen.
Here are a couple of examples of this.
Example 1:You want to express your chances of rolling a six on a six
sided die in odds format. There are five ways to get something
other than a six, and only one way to get a six, so the odds are
5 to 1.
You want to express the odds of drawing an ace of spades out
a deck of cards. 51 of those cards are something else, but one
of those cards is the ace, so the odds are 51 to 1.
Odds become useful when you compare them with payouts on
bets. True odds are when a bet pays off at the same rate as its
probability.
Here’s an example of true odds:
You and your buddy are playing a simple gambling game you
made up. He bets a dollar on every roll of a single die, and he
gets to guess a number. If he’s right, you pay him $5. If he’s
wrong, he pays you $1.
Since the odds of him winning are 5 to 1, and the payoff is
also 5 to 1, you’re playing a game with true odds. In the long
run, you’ll both break even. In the short run, of course,
anything can happen.
Probability and Expected Value
One of the truisms about probability is that the greater the
number of trials, the closer you’ll get to the expected results.
If you changed the equation slightly, you could play this
game at a profit. Suppose you only paid him $4 every time he
won. You’d have him at an advantage, wouldn’t you?
Blackjack Single Deck Strategy
- He’d win an average of $4 once every six rolls
- But he’d lose an average of $5 on every six rolls
- This gives him a net loss of $1 for every six rolls.
You can reduce that to how much he expects to lose on every
single roll by dividing $1 by 6. You’ll get 16.67 cents.
On the other hand, if you paid him $7 every time he won, he’d
have an advantage over you. He’d still lose more often than he’d
win. But his winnings would be large enough to compensate for
those 5 losses and then some.
The difference between the payout odds on a bet and the true
odds is where every casino in the world makes its money. The
only bet in the casino which offers a true odds payout is the
odds bet in craps, and you have to make a bet at a disadvantage
before you can place that bet.
Here’s an actual example of how odds work in a casino. A
roulette wheel has 38 numbers on it. Your odds of picking the
correct number are therefore 37 to 1. A bet on a single number
in roulette only pays off at 35 to 1.
You can also look at the odds of multiple events occurring.
The operative words in these situations are “and” and “or”.
- If you want to know the probability of A happening AND
of B happening, you multiply the probabilities. - If you want to know the probability of A happening OR of
B happening, you add the probabilities together.
Here are some examples of how that works.
Example 1:You want to know the probability that you’ll draw an ace of
spades AND then draw the jack of spades. The probability of
drawing the ace of spades is 1/52. The probability of then
drawing the jack of spades is 1/51. (That’s not a typo—you
already drew the ace of spades, so you only have 51 cards left
in the deck.)
The probability of drawing those 2 cards in that order is
1/52 X 1/51, or 1/2652.
You want to know the probability that you’ll get a blackjack.
That’s easily calculated, but it varies based on how many decks
are being used. For this example, we’ll use one deck.
To get a blackjack, you need either an ace-ten combination,
or a ten-ace combination. Order doesn’t matter, because either
will have the same chance of happening.
Your probability of getting an ace on your first card is
4/52. You have four aces in the deck, and you have 52 total
cards. That reduces down to 1/13.
Your probability of getting a ten on your second card is
16/51. There are 16 cards in the deck with a value of ten; four
each of a jack, queen, king, and ten.
So your probability of being dealt an ace and then a 10 is
1/13 X 16/51, or 16/663.
The probability of being dealt a 10 and then an ace is also
16/663.
You want to know if one or the other is going to happen, so
you add the two probabilities together.
16/663 + 16/663 = 32/663.
That translates to approximately 0.0483, or 4.83%. That’s
about 5%, which is about 1 in 20.
You’re playing in a single deck blackjack game, and you’ve
seen 4 hands against the dealer. In all 4 of those hands, no ace
or 10 has appeared. You’ve seen a total of 24 cards.
What is your probability of getting a blackjack now?
Your probability of getting an ace is now 4/28, or 1/7.
(There are only 28 cards left in the deck.)
Your probability of getting a 10 is now 16/27.
Your probability of getting an ace and then a 10 is 1/7 X
16/27, or 16/189.
Again, you could get a blackjack by getting an ace and a ten
or by getting a ten and then an ace, so you add the two
probabilities together.
16/189 + 16/189 = 32/189
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Your chance of getting a blackjack is now 16.9%.
This last example demonstrates why counting cards works. The
deck has a memory of sorts. If you track the ratio of aces and
tens to the low cards in the deck, you can tell when you’re more
likely to be dealt a blackjack.
Since that hand pays out at 3 to 2 instead of even money,
you’ll raise your bet in these situations.
The House Edge
The house edge is a related concept. It’s a calculation of
your expected value in relation to the amount of your bet.
Here’s an example.
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5%.
Expected value is just the average amount of money you’ll win
or lose on a bet over a huge number of trials.
Using a simple example from earlier, let’s suppose you are a
12 year old entrepreneur, and you open a small casino on the
street corner. You allow your customers to roll a six sided die
and guess which result they’ll get. They have to bet a dollar,
and they get a $4 win if they’re right with their guess.
Over every six trials, the probability is that you’ll win
five bets and lose one bet. You win $5 and lose $4 for a net win
of $1 for every 6 bets.
Your house edge is 16.67% for this game.
The expected value of that $1 bet, for the customer, is about
84 cents. The expected value of each of those bets–for you–is
$1.16.
That’s how the casino does the math on all its casino games,
and the casino makes sure that the house edge is always in their
favor.
With blackjack, calculating this house edge is harder. After
all, you have to keep up with the expected value for every
situation and then add those together. Luckily, this is easy
enough to do with a computer. We’d hate to have to work it out
with a pencil and paper, though.
What does the house edge for blackjack amount to, then?
It depends on the game and the rules variations in place. It
also depends on the quality of your decisions. If you play
perfectly in every situation—making the move with the highest
possible expected value—then the house edge is usually between
0.5% and 1%.
If you just guess at what the correct play is in every
situation, you can add between 2% and 4% to that number. Even
for the gambler who ignores basic strategy, blackjack is one of
the best games in the casino.
Expected Hourly Loss and/or Win
You can use this information to estimate how much money
you’re liable to lose or win per hour in the casino. Of course,
this expected hourly win or loss rate is an average over a long
period of time. Over any small number of sessions, your results
will vary wildly from the expectation.
Here’s an example of how that calculation works.
- You are a perfect basic strategy player in a game with a
0.5% house edge. - You’re playing for $100 per hand, and you’re averaging
50 hands per hour. - You’re putting $5,000 into action each hour ($100 x 50).
- 0.5% of $5,000 is $25.
- You’re expected (mathematically) to lose $25 per hour.
Here’s another example that assumes you’re a skilled card
counter.
- You’re able to count cards well enough to get a 1% edge
over the casino. - You’re playing the same 50 hands per hour at $100 per
hand. - Again, you’re putting $5,000 into action each hour ($100
x $50). - 1% of $5,000 is $50.
- Now, instead of losing $25/hour, you’re winning $50 per
hour.
Effects of Different Rules on the House Edge
The conditions under which you play blackjack affect the
house edge. For example, the more decks in play, the higher the
house edge. If the dealer hits a soft 17 instead of standing,
the house edge goes up. Getting paid 6 to 5 instead of 3 to 2
for a blackjack also increases the house edge.
Luckily, we know the effect each of these changes has on the
house edge. Using this information, we can make educated
decisions about which games to play and which games to avoid.
Here’s a table with some of the effects of various rule
conditions.
Single Deck Blackjack
Rules Variation | Effect on House Edge |
---|---|
6 to 5 payout on a natural instead of the stand 3 to 2 payout | +1.3% |
Not having the option to surrender | +0.08% |
8 decks instead of 1 deck | +0.61% |
Dealer hits a soft 17 instead of standing | +0.21% |
Player is not allowed to double after splitting | +0.14% |
Player is only allowed to double with a total of 10 or 11 | +0.18% |
Player isn’t allowed to re-split aces | +0.07% |
Player isn’t allow to hit split aces | +0.18% |
These are just some examples. There are multiple rules
variations you can find, some of which are so dramatic that the
game gets a different name entirely. Examples include Spanish 21
and Double Exposure.
The composition of the deck affects the house edge, too. We
touched on this earlier when discussing how card counting works.
But we can go into more detail here.
Every card that is removed from the deck moves the house edge
up or down on the subsequent hands. This might not make sense
initially, but think about it. If you removed all the aces from
the deck, it would be impossible to get a 3 to 2 payout on a
blackjack. That would increase the house edge significantly,
wouldn’t it?
Here’s the effect on the house edge when you remove a card of
a certain rank from the deck.
Card Rank | Effect on House Edge When Removed |
---|---|
2 | -0.40% |
3 | -0.43% |
4 | -0.52% |
5 | -0.67% |
6 | -0.45% |
7 | -0.30% |
8 | -0.01% |
9 | +0.15% |
10 | +0.51% |
A | +0.59% |
These percentages are based on a single deck. If you’re
playing in a game with multiple decks, the effect of the removal
of each card is diluted by the number of decks in play.
Looking at these numbers is telling, especially when you
compare these percentages with the values given to the cards
when counting. The low cards (2-6) have the most dramatic effect
on the house edge. That’s why almost all counting systems assign
a value to each of them. The middle cards (7-9) have a much
smaller effect. Then the high cards, aces and tens, also have a
large effect.
The most important cards are the aces and the fives. Each of
those cards is worth over 0.5% to the house edge. That’s why the
simplest card counting system, the ace-five count, only tracks
those two ranks. They’re that powerful.
You can also look at the probability that a dealer will bust
based on her up card. This provides some insight into how basic
strategy decisions work.
Dealer’s Up Card | Percentage Chance Dealer Will Bust |
---|---|
2 | 35.30% |
3 | 37.56% |
4 | 40.28% |
5 | 42.89% |
6 | 42.08% |
7 | 25.99% |
8 | 23.86% |
9 | 23.34% |
10 | 21.43% |
A | 11.65% |
Perceptive readers will notice a big jump in the probability
of a dealer busting between the numbers six and seven. They’ll
also notice a similar division on most basic strategy charts.
Players generally stand more often when the dealer has a six or
lower showing. That’s because the dealer has a significantly
greater chance of going bust.
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Summary and Further Reading
Odds and probability in blackjack is a subject with endless
ramifications. The most important concepts to understand are how
to calculate probability, how to understand expected value, and
how to quantify the house edge. Understanding the underlying
probabilities in the game makes learning basic strategy and card
counting techniques easier.
We begin this Blackjack guide with a very important assertion – casino games, hence the name, are designed with a built-in house advantage – a casino's edge in odds over a player at any given bet.
Thus, by stepping into a casino, you know that the odds by default are stacked against you to a certain degree. This means that in the long term you will always lose money, unless you skew the odds to even or in your favor.
Casinos believe that as soon as you enter their premises, it is just a matter of time before all your money becomes theirs. And since they want to keep you around for as long as possible, they will attract you with the so-called comps – complimentary items and services given out by the house to encourage players to gamble.
Casinos will do everything they can to make you feel cozy, valued and inclined to prolong your stay at the tables. These “freebies” they give out depend on the game a person plays, for how long, and of course on the size of his bets. Casinos have staff whose job is to manage the comps and contact players to persuade them to visit and play.
As a casino game, Blackjack also falls under this principle and is designed to transfer the money from your pocket into the house's vault, unless some method of advantage play is used. This is where this guide comes in.
We will present ideas which will can help you shift odds to near-even, and in some cases even slightly in your favor. This will be done in the first section of the guide where we will discuss strategies relating to the basic strategy player.
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Probability Of Blackjack Single Deck
Later on we will move to the so-called advantage player, where card counting takes place. Whereas the basic strategy player is on relatively even ground with the house, albeit still behind, the advantage players, hence the name, have an advantage over the casino as soon as the game begins.
Regardless of which type of player you want to become, both require the aforementioned “method of advantage play” – strategies, card counting etc., which help skew the odds out of the casino's favor.
Is gambling a sin desiring god. Blackjack is one of the most widely played casino games for several reasons – it has simple rules, it is offered by almost every casino and most importantly – offers the best overall odds for the player, even for the non-advtantage one. It is simply a game that falls under the principles of odds and probabilities and can be beaten through lots of experience, knowledge and skill.
Odds and probabilities
Now, one would wonder how come if the stacks are always stacked against the non-advantage players, who make up most of the player mass, there are still many people going to the casinos. After all, if all these people were constantly losing, they would ultimately become discouraged and the majority will stop playing.
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Gamblers hope that on any particular night they might be lucky and the odds fall in their favor. For example, although tossing a coin has a 50% chance to turn heads or tails, when you toss it only 20 times, it might land 14 times on heads and 6 on tails. The even probability of tossing tails or heads is true only when you repeat the exercise enough times to ensure a large enough sample pool.
And because many people get lucky at any given moment, they will earn money, fueling their excitement and luring them back in the casino the next night. And although this causes the house to lose money, the casino relies on the fact that players place millions of bets, which irons out the outcome in the long-term and makes the house profitable. In mathematics, this is known as the law of large numbers.
This rule dictates that the more bets are placed, the closer the casino's odds will come to the artificially created built-in house advantage, thus the more hands (bets) a non-advantage gambler plays (places), the smaller his chances to win become.
Thus, players who do not employ advantage methods will be losing money if they continue to play in the long-term, which allows casinos to continue to operate, while basic strategy and advantage players profit from their efforts.
Blackjack Rules
Additional Blackjack Rules and Variations
Blackjack Basic Strategy
Universal Strategy for Blackjack
Splitting Pairs
Calculating the house's advantage in blackjack is difficult due to the huge amount of card combinations and also because not all winning bets have the same payout (for example, a blackjack which is a 21-hand with an ace and a 10-valued card, pays 3:2 in most casinos).
The extent and source of the house's advantage varies in different casinos. In blackjack, for example, it is derived from the fact that when the player busts himself by exceeding the count of 21, the house wins the bet, regardless of whether the dealer would also bust when drawing more cards to complete the hand for other players on the table.
Improving or reducing the house's advantage
The house edge depends on the rules the casino has established for the blackjack table and they are posted and easily visible either on, or near the table.
For example, a casino where dealers stand on a soft 17 (S17), instead of hitting it (H17), favors the players' odds, reducing the house's edge by around 0.2%. Increasing the number of decks also increases the house's edge, and vice versa.
The implementation of other rules such as the allowance or disallowance of resplitting, doubling down after split, double on 9/10/11 or only on 10/11 and so on. Casinos typically aim to boost their advantage but also not to discourage players from joining the game and, thus, seek some form of balance. For example, if they offer one-deck blackjack, they typically forbid doubling on soft hands or after splitting, restrict resplitting and so on.
Number of Decks | House Advantage |
---|---|
Single deck | 0.17% |
Double deck | 0.46% |
Four decks | 0.60% |
Six decks | 0.64% |
Eight decks | 0.65% |
The table above illustrates the house's edge relative to the different number of decks, considering the following set of rules:
– dealer hits soft 17
– double split is allowed
– resplit to four hands is allowed
– no surrender
– no hitting split aces
– double on any two cards
– original bets only lost on dealer blackjack
– cut-card used
As evident, the house's edge experiences the most dramatic jump when raising the number of decks from one to two and grows progressively slowerr as more decks are added.
Hand-held and shoe games, house edge
Single-deck games belong to the so-called hand-held games. Double-deck games could also be hand-held. Although they are still being offered by some casinos, their peak of popularity has passed. Nowadays, casinos more often host shoe games.
In a shoe game, a card shoe (rectangular box) holds multiple decks of cards. The dealer pulls cards out of the shoe and slides them to the appropriate areas in front of the players. Cards are most often dealt face up, which prohibits the player from touching them. Even if the gambler splits their hand, they don't touch the cards and instead just place their additional bet and the dealer does the splitting.
Only in games where cards are dealt facing down can the player hold his cards, but only in one hand.
Almost always you will see shoe games consist of multiple decks, four and more, and on rare occurrences two. Thus, shoe games will logically favor the house's odds.
You may also encounter tables using the so-called Continuous-Shuffling Machines. These devices hold three or more decks and after a couple of rounds the dealer returns the used cards to the machine and shuffles them with the unused cards. Thus, each shuffle refreshes the shoe, making it impossible to count cards, which is why card counters avoid these games.
As for basic strategy players, CSMs don't have such an impact, but they increase the number of hands played per hour, meaning the player will incur larger money losses (why we will explain later).