Rule of 2 and 4 (poker)

Outs × 2 is the chance on the next card; × 4 is the chance by the river.

A 9-out flush draw is 18% with × 2 and 36% with × 4 (35% after the correction). But use × 4 only when you are all-in on the flop and see both cards for free. Normally one call buys only one card, so use × 2.