Pot odds are the cheapest edge in poker and the most commonly miscounted. The question is only ever: does the chance of getting there beat the price of finding out?
Put the pot and the bet in, then your outs. It gives the equity you need and the equity you have.
Nine for a flush draw, eight for an open-ended straight, four for a gutshot, fifteen for a straight-and-flush draw. Subtract any out that makes your hand and a better one for somebody else — a flush draw on a paired board is not nine clean outs.
Multiplying outs by four on the flop assumes you always see both cards. You do not, unless you are all-in. Facing a bet you expect to be followed by another one, use the one-card number twice rather than the two-card number once.
The calculator prices the pot as it stands. If hitting wins you another two streets, the call is better than the number says; if your opponent shuts down the moment the flush comes in, it is worse.
The price of a call as a share of the pot you would win. Calling 50 into a pot of 100 costs you a third of the final pot, so you need to win a third of the time to break even.
Nine. Thirteen cards of each suit, four of them already visible — two in your hand and two on the board.
Close enough at the table, and consistently a little optimistic. The tool shows both so you can see the gap.