I bought a large and medium, and both fit almost the same. it feels great while swimming, like it's not even there.

I love this rashguard.  The first one I ordered the sleeves were too short although it was the same size as this one - large.  I usually wear a size small or medium in a shirt but rashguards are so snug that I bought a large.  I got the yellow one, it is very attractive on.  I used it snorkeling in Mexico.  It was great not to have to apply sun screen all the time.