The rule of 2 and 4

The rule

Count your outs — cards that you believe make you the best hand. On the flop, multiply by four to estimate equity through the river. On the turn, multiply by two to estimate equity to the river. Nine-out flush draw: ~36% flop to river, ~18% turn to river. Eight-out open-ended straight: ~32% and ~16%. Two-out set: ~8% and ~4%.

Where it lies

It slightly overcounts (the real multiplier is closer to 4.1 / 2.2 on very large out counts it overshoots more). It assumes every out is clean. It ignores the times you hit and lose anyway. Discount dirty outs — the fourth flush card that also completes a bigger flush, the straight card that pairs the board. A '12-out' draw with four dirty outs is an 8-out draw with a story.

It also assumes you will see the card for free. If they can bet the turn and you will have to fold, flop-to-river equity is not the number you own. Price the street in front of you first.