I typically would wear a medium-size shirt. I followed some previous recommendations and bought two sizes up and got the XL size. I was glad that I did. It fit perfectly -- just tight enough. I have a slender build so it worked just great. The dark blue color that I picked was stylish-looking (I felt a bit like a superhero in the tight shirt) and I had a noticeable difference in terms of the amount of sun exposure my skin got when covered by the shirt. I haven't used it for snorkeling but I spent a good amount of time on the beach and in the ocean and saw no considerable fading or wear and tear.