take the number of outs, multiply by 2 and add 1

For example, if you hold [Ah] [Kh]

and the flop is

[9h] [10h] [Js]

you can catch any heart (9), or any queen (4) to have a hand worth betting... 13 outs, X 2 + 1 = 27% odds of improving

this means, roughly, 3.7 to 1 so, the pot must be 3.7 times whatever bet you face (round it to 4 if you wish for simplicity)..

Hope this helps