The shirt is extremely lightweight and comfortable, however it runs pretty big. I am 5'10" 185 muscular build. I wear a size large in just about every brand, TNF, Patagonia etc. I initially ordered a Large in the Echo, but it was way too big on me and my fit preferences so I had to return it for a medium. The medium Echo fits looser than a Large Patagonia Nine Trails shirt. The closest size comparison I can come up with is Under Armour. If you own any of the basic Under Armour charged cotton t-shirts it fits about the same as those.