the XL fit my son who usually wears a large so I'd go a size up if in doubt. Was great for our cruise for our poor pigment-challenged children.  They only had to put sunscreen on their face and lower legs if they wore these with their swim trunks.  Our then 15 year old soon deemed this "cool", so take that for what it's worth.  Was nice especially for snorkeling as his back was to the sun all day.