Pot odds
The price of a call: what you pay versus what the pot pays back.
You're on the button with 9♣8♣ and the flop comes K♣7♣2♦ — two clubs, so you're on a flush draw. The turn is the 4♥, no help. Your opponent bets $40 into $100. You call $40 to win $140: that's 140-to-40, about 3.5-to-1, so you need roughly 22 percent equity to break even. A bare flush draw has nine clubs left, 9 outs in 46 unseen cards, about 19.6 percent — a hair short of what the pot asks. Give that same draw a gutshot and you're at 12 outs, past 26 percent, clear of the price.
→ $10 to win $30 → you need 25% equity to break even.
This turns a hunch into a number. Instead of guessing whether you'll hit, you check whether the price beats your chance of hitting. Quick estimate: the rule of 2 and 4. Multiply your outs by 4 with two cards to come, by 2 with one card, then set that against what the pot lays you. Do it on every draw and you stop calling bets that look juicy but pay less than they cost.
The usual mistake is calling because a draw feels alive, without ever pricing it. A gutshot is 4 outs, about 17 percent with two cards to come, and players fire calls with it anyway, chasing a hand the pot isn't paying for. The other trap is counting outs that don't win — a flush draw against a board where someone already has a full house fills nothing. Count only the outs that actually take the pot.