Velvet RiverRule of 2 and 4
A shortcut to estimate your chance to hit a draw: outs × 2 for one card, outs × 4 for two cards.
With 9 outs on the flop: 9 × 4 = 36%, close to the true 35%. On the turn: 9 × 2 = 18%, close to the true 19.6%. The rule overestimates slightly when you have more than about 10 outs, so trim a few points there.