Rule of two and four
The rule of two and four is a shortcut for a drawing hand: multiply your outs by two for the chance of hitting on the next card, and by four for the chance of hitting by the river.
It is close enough to use at the table and it is not exact. Nine outs gives 18% and 36% by the shortcut against real figures of 19.1% and 35.0%, so the first number is slightly low and the second slightly high.
The error grows with the number of outs, and only in the four version. With 15 outs the shortcut says 60% while the true figure is 54.1%, so with a very large draw it is worth doing the sum properly rather than trusting the doubling.
an example
your cards
board
Nine hearts are left, so the shortcut gives 18% on the turn and 36% by the river. The true numbers are 19.1% and 35.0%, which is close enough to decide whether a bet of half the pot is worth calling.
This example is made up. The real one is in your own hand history: find rule of two and four in your own hands and see how often it came up and what it cost you.
related terms
Outs
An out is a card still in the deck that would give you the best hand, and counting them is how you turn a draw into a nu ...
Equity
Your equity in a pot is the share of it you would win on average if the remaining cards were dealt out and nobody folded ...
Pot odds
Pot odds are the price you are offered on a call: what you have to put in compared with what the pot would be worth to y ...
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Knowing the word is not the same as seeing it.
Forty hands against a bot, no account, and your own hand history at the end.