A straight flush is possible whenever the board contains at least three cards of the same suit where the ranks of the suited cards can create a straight with the addition of exactly two ranks. A Straight can be found in the middle of the poker hand rankings. It is weaker than a Flush and stronger than Three of a Kind. A Straight card hand is also ranked based on the value of the. The Probability of a Straight Flush (fraction) constant defines the probability of being dealt a Straight Flush and represent the probability as a fraction. The Straight Flush is a five card hand having five successive value cards all in the same suit exactly. There are 40 straight flushes (ten in each suit, from the “wheel” A-2–3–4–5 to the “royal” 10-J-Q-K-A.) The total number of five card hands is 52C5 = 52! 5!) = 2,598,960. So the probability of getting dealt a straight flush in five cards is 40 / 2,598,960 = 1 / 64,974 = 1.5391E-05.
In poker, the probability of each type of 5-card hand can be computed by calculating the proportion of hands of that type among all possible hands.
The following enumerates the (absolute) frequency of each hand, given all combinations of 5 cards randomly drawn from a full deck of 52 without replacement. Wild cards are not considered. The probability of drawing a given hand is calculated by dividing the number of ways of drawing the hand by the total number of 5-card hands (the sample space, five-card hands). The odds are defined as the ratio (1/p) - 1 : 1, where p is the probability. Note that the cumulative column contains the probability of being dealt that hand or any of the hands ranked higher than it. (The frequencies given are exact; the probabilities and odds are approximate.)
The nCr function on most scientific calculators can be used to calculate hand frequencies; entering nCr with 52 and 5, for example, yields as above.
Hand | Frequency | Approx. Probability | Approx. Cumulative | Approx. Odds | Mathematical expression of absolute frequency |
---|---|---|---|---|---|
Royal flush | 4 | 0.000154% | 0.000154% | 649,739 : 1 | |
Straight flush (excluding royal flush) | 36 | 0.00139% | 0.00154% | 72,192.33 : 1 | |
Four of a kind | 624 | 0.0240% | 0.0256% | 4,164 : 1 | |
Full house | 3,744 | 0.144% | 0.170% | 693.2 : 1 | |
Flush (excluding royal flush and straight flush) | 5,108 | 0.197% | 0.367% | 507.8 : 1 | |
Straight (excluding royal flush and straight flush) | 10,200 | 0.392% | 0.76% | 253.8 : 1 | |
Three of a kind | 54,912 | 2.11% | 2.87% | 46.3 : 1 | |
Two pair | 123,552 | 4.75% | 7.62% | 20.03 : 1 | |
One pair | 1,098,240 | 42.3% | 49.9% | 1.36 : 1 | |
No pair / High card | 1,302,540 | 50.1% | 100% | .995 : 1 | |
Total | 2,598,960 | 100% | 100% | 1 : 1 |
The royal flush is a case of the straight flush. It can be formed 4 ways (one for each suit), giving it a probability of 0.000154% and odds of 649,739 : 1.
When ace-low straights and ace-low straight flushes are not counted, the probabilities of each are reduced: straights and straight flushes each become 9/10 as common as they otherwise would be. The 4 missed straight flushes become flushes and the 1,020 missed straights become no pair.
Note that since suits have no relative value in poker, two hands can be considered identical if one hand can be transformed into the other by swapping suits. For example, the hand 3♣ 7♣ 8♣ Q♠ A♠ is identical to 3♦ 7♦ 8♦ Q♥ A♥ because replacing all of the clubs in the first hand with diamonds and all of the spades with hearts produces the second hand. So eliminating identical hands that ignore relative suit values, there are only 134,459 distinct hands.
The number of distinct poker hands is even smaller. For example, 3♣ 7♣ 8♣ Q♠ A♠ and 3♦ 7♣ 8♦ Q♥ A♥ are not identical hands when just ignoring suit assignments because one hand has three suits, while the other hand has only two—that difference could affect the relative value of each hand when there are more cards to come. However, even though the hands are not identical from that perspective, they still form equivalent poker hands because each hand is an A-Q-8-7-3 high card hand. There are 7,462 distinct poker hands.
of the binomial coefficients and their interpretation as the number of ways of choosing elements from a given set. See also: sample space and event (probability theory).
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