I'm 5'6" 160 pounds. I bought a medium because that seems to be the average size for almost everything I wear. Turns out it fit perfectly as expected. I could have bought the small for a tighter fit but because I'm currently working on packing more overall muscle and I naturally have a much rounder rear end than most guys lol I rather wear clothes that give me more room to breath from the front and back. Anyways my only gripe (and it's not a big deal) is the fact that these pants don't have back pockets. I prefer pants with pockets. But overall I absolutely recommend buying a pair if not more because the quality, comfort, and thin but strong material are quite impressing and there's elasticity so it compliments your physique gains from the gym.