I wear a 10 1/2 in casual shoes. My foot is medium width. And in sandals, I usually find that the fit is too loose across the top in the forefoot.  Flip flops in a 10 usually fit me. I used to buy Reef brand. Now I am a fan of the Cobian Bounce. The size 10 fits me perfectly. I love and need the arch support. And I love the coushion-y feel. I will buy this in other colors.