I got these for my husband who works in an un-heated auto shop. I got him the Large size because that's mostly what he wears in all brands. He's about 6' tall and medium build.  These thermal pants are very comfortable and warm, but the fit is strange. They're mostly tight (comfortably), but so loose in the waist that he has elastic bunched up underneath his belt. The fit is so far off on these I wouldn't even know what might fit better other than completely altering them. Medium would probably be perfect in the waist but too small everywhere else, while Large is mostly good except for the bunched up elastic in the waist. He only wears them if all his other thermals are dirty.