multi handed odds

I know this is probably in the forums somewhere already, but i really dont have time to search right now, so im just posting it anyway.
I know the 'heads up' winning percentages for certain hands before the flop, and its easy to figure it out after the flop, but how do you figure out ur winning percentages against multiple opponents, both before the flop and after the flop? It would be helpful to know because when im playing i know what my chances are against a single opponent and can make a good geuss on what my winning chances are preflop based on what i think he might have, but when there are 2 or more people in the pot, i have no idea what my chances are. If anyone could give me a method or formula it would be greatly appreciated.