You’re on a flush draw with two cards to come. How often do you hit? Count your OUTS — the cards that make your hand — and the ‘rule of 4 and 2’ is close: about 4% per out with two cards left. Nine outs is roughly 35%. Then compare that to the POT ODDS the bet is offering: call only if the price is right. Slide your outs and read the equity.
With O outs among 47 unseen cards (after the flop) and two cards to come, the chance to hit is 1 − (47−O)/47 · (46−O)/46 — a hypergeometric (draw-without-replacement) calculation. The ‘rule of 4’ approximates it as 4·O% (and 2·O% for one card). Nine outs (a flush draw) is 1 − (38/47)(37/46) ≈ 35%. You should CALL when your equity exceeds the pot odds (call size / final pot). A fail-loud self-check throws unless 9 outs yields ~35% by the river. ◆ real probability, node-verified.
The hypergeometric equity is exact for known outs; real decisions also weigh implied odds, opponents still to act, and whether an ‘out’ truly wins (a flush card can complete a better hand). The number is your DRAW equity, not your win certainty.