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pokerstove is perhaps the easiest way to calculate these, but if you're looking for the actual mathematical formula to use then hope you have alot of time on your hands.
For a simple case, lets say we have KK, and our opponent has AA. Lets also say that we have a board of 2s8cTh4d. What percent of the time will KK win the hand? Well, our rule of 2 says that we have about a 4% chance of hitting a K...why?
There are 44 cards left in the deck, and 2 of them are kings. To win, we need to hit one. Expressed in choose notation, the probability we win is
2 choose 1 / 44 choose 1
or the chance we hit over the total number. With only 1 card to come, this simplifies to 2/44 = 1/22 = 4.5%. Pokerstove gives the same value.
Ok, now suppose the board is instead 2s8cTh, now what is our percent chance to win? Rule of 4 says about 8%. Why?
still 2 kings, but since there are now two cards we have to consider a bunch of different scenarios. I picked a board that makes straights and flushes impossible since they are a bit more complicated to figure out, if your interested i challenge you to figure that out .
Right, so a bunch of different scenarios. The combos of turn and rivers we have are
KK
K-, KA
--
where a dash represents neither a k or A hitting. We have to consider the case where an A hits and a King hits due to that actually being a loss. Ok, so now how often does each thing happen? Since there are no flushes or straights, we can ignore -- and KA as we are only interested in the percent chance we win.
First case: 2 kings, we want both and two cards to come. 7 cards are known, 45 left in the deck.
2 c 2 / 45 c 2 = 1/990 = .0010 = .1%
Second case: 2 kings, want 1, 41 cards thar arent kings or aces
2c1*41c1 / 45c2 = 82/990 = .0828 = 8.28%
In all other scenarios, we lose. Sum it up
.1% + 8.28% = 8.29% chance KK will beat AA with a board of 2s8cTh. Pokerstove says 8.3%. Figuring out preflop percentages work out in a similar fashion, but there are alot more possible scenarios including those with straights and flushes so it gets a bit messy. That is the math that goes on to determine % chance to win against a single hand. As you can see though, its virtually impossible to do this kind of thing correctly at the table, so pokerstove is awesome.
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