Outs are the unseen cards that complete your hand — the cards that turn a draw into the best hand. Counting them, and converting the count into a percentage, is how you measure a draw’s equity against the price of a call.
Count outs before you call with any draw. A flush draw has 9 outs, an open-ended straight draw (OESD) has 8, a gutshot has 4, and a flush draw plus an OESD has about 15. Convert with the 2-and-4 rule: about 2% per out for the next card, about 4% per out when two cards are still to come and all the money goes in now. So a gutshot is about 8% on the next card, and a combo draw with 15 outs is about 33% with one card to come.
Not every out is clean. A card that completes your straight can also put four to a straight on the board or give villain a better hand; those dirty outs are worth far less than they look. On J♥T♦8♣K♠ with Q♣7♥ facing 10 into 14, a nine puts four to a straight on board and an ace gives any queen the same straight, so the outs are worth far less than their count — the course folds. Count only clean outs, and fold when the outs you were counting are no longer good.
Worked example
FAQ
How many outs does a flush draw have?
Nine. There are 13 cards of each suit; with two in your hand and two on the board, nine are left among the unseen cards. That is about 19% on the next card and about 35% by the river.
What are dirty outs?
Cards that complete your draw but also improve villain or make your hand second best — for example a straight card that puts four to a straight on the board. Discount them, or ignore them, when you compare equity with the price.