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This post works with 5-card Poker hands drawn from a standard deck of 52 cards. The discussion is mostly mathematical, using the Poker hands to illustrate counting techniques and calculation of probabilities

The game of poker is a card game played among two or more players for several rounds. There are several varieties of the game, but they all tend to have these aspects in common: The game begins with each player putting down money allocated for betting. During each round of play, players are dealt cards from a standard 52-card deck, and the goal of each player is to have the best 5-card hand at. Poker Hand rankings. 2: $0.05 per $1 pot: $5: 3 - 4: $0.05 per $1 pot: $10: 5 - 10: $0.05 per $1 pot: $20: Short.

• The difference from normal poker is that Aces are always high, so that A-2-3-4-5 is not a straight, but ranks between K-Q-J-10-8 and A-6-4-3-2. The best hand in this form is 7-5-4-3-2 in mixed suits, hence the name 'deuce to seven'.
• Seriously, there is basically no discernible skill barrier between 1/2 and 2/5. 2/5 players think their better, and I guess they are on some level, but it's still terrible live poker. You may find one other decent player at a 2/5 (although I have played plenty of tables with 9 awful fish), but mostly it's loose passive players or rocks.

Working with poker hands is an excellent way to illustrate the counting techniques covered previously in this blog – multiplication principle, permutation and combination (also covered here). There are 2,598,960 many possible 5-card Poker hands. Thus the probability of obtaining any one specific hand is 1 in 2,598,960 (roughly 1 in 2.6 million). The probability of obtaining a given type of hands (e.g. three of a kind) is the number of possible hands for that type over 2,598,960. Thus this is primarily a counting exercise.

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Preliminary Calculation

Usually the order in which the cards are dealt is not important (except in the case of stud poker). Thus the following three examples point to the same poker hand. The only difference is the order in which the cards are dealt.

These are the same hand. Order is not important.

The number of possible 5-card poker hands would then be the same as the number of 5-element subsets of 52 objects. The following is the total number of 5-card poker hands drawn from a standard deck of 52 cards.

The notation is called the binomial coefficient and is pronounced “n choose r”, which is identical to the number of -element subsets of a set with objects. Other notations for are , and . Many calculators have a function for . Of course the calculation can also be done by definition by first calculating factorials.

Thus the probability of obtaining a specific hand (say, 2, 6, 10, K, A, all diamond) would be 1 in 2,598,960. If 5 cards are randomly drawn, what is the probability of getting a 5-card hand consisting of all diamond cards? It is

This is definitely a very rare event (less than 0.05% chance of happening). The numerator 1,287 is the number of hands consisting of all diamond cards, which is obtained by the following calculation.

The reasoning for the above calculation is that to draw a 5-card hand consisting of all diamond, we are drawing 5 cards from the 13 diamond cards and drawing zero cards from the other 39 cards. Since (there is only one way to draw nothing), is the number of hands with all diamonds.

If 5 cards are randomly drawn, what is the probability of getting a 5-card hand consisting of cards in one suit? The probability of getting all 5 cards in another suit (say heart) would also be 1287/2598960. So we have the following derivation.

Thus getting a hand with all cards in one suit is 4 times more likely than getting one with all diamond, but is still a rare event (with about a 0.2% chance of happening). Some of the higher ranked poker hands are in one suit but with additional strict requirements. They will be further discussed below.

Another example. What is the probability of obtaining a hand that has 3 diamonds and 2 hearts? The answer is 22308/2598960 = 0.008583433. The number of “3 diamond, 2 heart” hands is calculated as follows:

One theme that emerges is that the multiplication principle is behind the numerator of a poker hand probability. For example, we can think of the process to get a 5-card hand with 3 diamonds and 2 hearts in three steps. The first is to draw 3 cards from the 13 diamond cards, the second is to draw 2 cards from the 13 heart cards, and the third is to draw zero from the remaining 26 cards. The third step can be omitted since the number of ways of choosing zero is 1. In any case, the number of possible ways to carry out that 2-step (or 3-step) process is to multiply all the possibilities together.

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The Poker Hands

Here’s a ranking chart of the Poker hands.

The chart lists the rankings with an example for each ranking. The examples are a good reminder of the definitions. The highest ranking of them all is the royal flush, which consists of 5 consecutive cards in one suit with the highest card being Ace. There is only one such hand in each suit. Thus the chance for getting a royal flush is 4 in 2,598,960.

Royal flush is a specific example of a straight flush, which consists of 5 consecutive cards in one suit. There are 10 such hands in one suit. So there are 40 hands for straight flush in total. A flush is a hand with 5 cards in the same suit but not in consecutive order (or not in sequence). Thus the requirement for flush is considerably more relaxed than a straight flush. A straight is like a straight flush in that the 5 cards are in sequence but the 5 cards in a straight are not of the same suit. For a more in depth discussion on Poker hands, see the Wikipedia entry on Poker hands.

The counting for some of these hands is done in the next section. The definition of the hands can be inferred from the above chart. For the sake of completeness, the following table lists out the definition.

Definitions of Poker Hands

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Counting Poker Hands

Straight Flush
Counting from A-K-Q-J-10, K-Q-J-10-9, Q-J-10-9-8, …, 6-5-4-3-2 to 5-4-3-2-A, there are 10 hands that are in sequence in a given suit. So there are 40 straight flush hands all together.

Four of a Kind
There is only one way to have a four of a kind for a given rank. The fifth card can be any one of the remaining 48 cards. Thus there are 48 possibilities of a four of a kind in one rank. Thus there are 13 x 48 = 624 many four of a kind in total.

Full House
Let’s fix two ranks, say 2 and 8. How many ways can we have three of 2 and two of 8? We are choosing 3 cards out of the four 2’s and choosing 2 cards out of the four 8’s. That would be = 4 x 6 = 24. But the two ranks can be other ranks too. How many ways can we pick two ranks out of 13? That would be 13 x 12 = 156. So the total number of possibilities for Full House is

Note that the multiplication principle is at work here. When we pick two ranks, the number of ways is 13 x 12 = 156. Why did we not use = 78?

Flush
There are = 1,287 possible hands with all cards in the same suit. Recall that there are only 10 straight flush on a given suit. Thus of all the 5-card hands with all cards in a given suit, there are 1,287-10 = 1,277 hands that are not straight flush. Thus the total number of flush hands is 4 x 1277 = 5,108.

Straight
There are 10 five-consecutive sequences in 13 cards (as shown in the explanation for straight flush in this section). In each such sequence, there are 4 choices for each card (one for each suit). Thus the number of 5-card hands with 5 cards in sequence is . Then we need to subtract the number of straight flushes (40) from this number. Thus the number of straight is 10240 – 10 = 10,200.

Three of a Kind
There are 13 ranks (from A, K, …, to 2). We choose one of them to have 3 cards in that rank and two other ranks to have one card in each of those ranks. The following derivation reflects all the choosing in this process.

Two Pair and One Pair
These two are left as exercises.

High Card
The count is the complement that makes up 2,598,960.

The following table gives the counts of all the poker hands. The probability is the fraction of the 2,598,960 hands that meet the requirement of the type of hands in question. Note that royal flush is not listed. This is because it is included in the count for straight flush. Royal flush is omitted so that he counts add up to 2,598,960.

Probabilities of Poker Hands

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2017 – Dan Ma

Hand rankings[edit]

The most fundamental of poker concern the hand rankings, because the hand rankings determine the winner. While betting is extremely important to the game, players are wagering on whether they have won, therefore a complete understanding of hand rankings must come first. These hand rankings do not apply to games played 'low', such as lowball or razz; see the section on 'low hands' below.

The cards are ranked thus, from low to high: 2, 3, 4, 5, 6, 7, 8, 9, 10, Jack, Queen, King, Ace. An ace is the highest card, but it can also function as the lowest in completing a straight. The two is usually called a 'deuce', and the three is sometimes called a 'trey'. Ten, Jack, Queen, King, and Ace are often abbreviated T, J, Q, K, and A, respectively, so that each card name has a single number or letter associated with it. This is commonly used in describing hands, for example, A-2-3-4-5 is a hand with an ace, a two ('deuce'), a three, a four, and a five — not necessarily in that order, but presenting them in that order makes it clear that the hand is a straight. A hand may also be written, say, A-A-x-x-x, where 'x' means any other card that does not form a better hand.

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A-2-3-4-5 is considered a five-high straight, and it is called a wheel or bicycle; this is the only time an ace plays as a low card. An ace-high straight flush is called a royal flush and it cannot be beaten. The only time it ties is when all 5 cards to the royal flush, i.e. A♥K♥Q♥J♥10♥, are on the community board. Higher cards always beat lower cards, for example, a pair of aces beats a pair of kings, and a flush with a king beats a flush whose highest card is a Queen. If two players have the same pair, a kicker is used to break the tie if possible (more about them soon). When two players have two pair, the highest pairs are considered, for example, aces up always beats kings up, no matter the other pairs. If, for example, two players both have aces up, then the higher of the smaller pairs wins: aces over kings beats aces over queens. If, for example, both players have aces over kings, then the kicker card is considered. Kickers also come into play when more than one player has the same three or four of a kind (possible only in community card games or wildcard games). If players have the same straight, flush, full house, or straight flush, it is always a tie and the players split the pot. There is no suit superiority or trump suit; a spade flush with A-10-9-6-4 does not beat a club flush with the same values.

A kicker is any card that you hold in your hand that does not make part of it, that is, an otherwise useless card. A hand can have more than one kicker; A pair for instance has three kickers and a three-of-a-kind has two, and they are considered in rank order highest-first. When two players hold the same pair, two pair, three of a kind, or four of a kind, the highest kicker wins, for example, A-A-K-x-x beats A-A-Q-x-x, A-A-K-Q-x beats A-A-K-J-x, and A-A-K-Q-J beats A-A-K-Q-T. A kicker can be higher than the rest of the hand, for example, K-K-A-x-x beats K-K-J-x-x, so an ace usually makes the best kicker. If the first kicker ties and there is a second or third, they are compared in rank order; A-A-K-J-x loses to A-A-K-Q-x. If the hands are totally equivalent, the pot is split.

Low hands[edit]

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Some games have a high-low split, and some games such as lowball or razz are played low-only. In a high-low split game, typically a low hand must not have any cards ranked higher than eight and no cards must be paired, or it does not count as a low hand. In low-only games, any cards can be used. Many forms of poker do not use low hands, so you need not concern yourself with these until you intend to play games that do.

There are three common ways of ranking low hands, ace-to-five low, ace-to-six low, and deuce-to-seven low, named after the best possible hands in the respective systems. In all systems, paired cards are bad and cannot be used to beat any hand that does not have a pair. Likewise, a pair beats three of a kind, three of a kind beats a full house, and a full house beats four of a kind. The most common hand ranking system for low hands is ace-to-five, used almost universally in high-low split games and very common in other games. This means A-2-3-4-5 (called a wheel or bicycle, just as it is as a high hand) is the best possible low hand, and the ace is the lowest card. For a high-low split game, it also forms a high hand: a five-high straight. In order to avoid confusion, we will discuss only ace-to-five low at the moment.

When pairs and any other 'bad' hands are not present, then the winner is the one whose highest card is lowest. For this reason, a low hand is usually described highest card first, to make it easier to tell which is lower. In ace-to-five, 8-4-3-2-A loses to 7-6-5-4-3 because the highest card in the first hand (eight) is higher than the highest card in the second hand (seven), even though all the other cards in the second hand are lower. If the highest cards are the same, then the next-highest cards are considered, and so on: 8-7-6-3-A loses to 8-7-5-4-2 because the second hand goes lower first.

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In ace-to-six low, straights and flushes count for high (that is, they're bad), and the best possible hand is A-2-3-4-6 unsuited, since it's the lowest possible card combination that avoids pairing, straights, and flushes. Deuce-to-seven is identical except the ace is the highest card, so the best possible hand is 2-3-4-5-7 unsuited. Therefore, in deuce-to-seven low, the hand that would make the worst possible high hand in traditional poker is the best possible low hand, and vice versa: a royal flush is the worst possible hand.

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