For your live play, you can just use the 4/2 rule. Take your total number of outs, a gutter for example. You've got 4 outs to a lock, but that's all. If there are two cards to come, and you're going to be all in, multiply number of outs by 4 to get the percentage the draw will hit. 4outs * 4 = 16%
To find the chance of the next card filling the draw, multiply by 2:
4*2 = 8%
These are not exact figures, but close enough for poker needs.
The other way is a bit longer, but more precise, and gets real fast in no time. On the flop there's 47 unseen cards, 4 are good, so 43 are bad 43:4 ~ 10.75:1
(This shows a true percentage of 8.5%)
The nice thing about this way of calculating your odds is that your answer is already in odds form. 10.75:1 are the expressed odds you need to make a call correct (not taking implied odds into account). In the first method attaining 8% is quite easy, but one could make an error converting 8% to the pot odds equivalent, which would be 11.5:1.