Pick your draw, or type in your own outs, and see the odds of hitting by the turn and by the river.
An out is any card left in the deck that makes your hand. A flush draw has nine outs: thirteen cards share your suit and you can see four of them, so nine remain. An open-ended straight draw has eight, a gutshot four.
To turn outs into odds, use the rule of 2 and 4. Multiply your outs by 2 for the chance to hit on the next card, and by 4 for the chance to hit by the river with both cards still to come. It slightly overstates the biggest draws, but it is close enough to use at the table.
Nine. Thirteen cards share your suit; you can see two in your hand and two on the flop, leaving nine. That is roughly 35% to complete by the river, or about 19% on the very next card.
Count the cards that improve you to a likely winner, without double-counting. Then multiply by 2 for the next card or by 4 for both cards to come. Eight outs is about 16% by the turn and 32% by the river.
Outs are the unseen cards that turn a losing hand into a winning one. If you hold four cards to a flush, the nine remaining cards of that suit are your outs. More outs means a better chance to hit.