I bought these for my son whos 12. He wears a mens 9.5 in tennis shoes. His foot is on the wider side with a high instep. A lot of flip flops don't fit for that reason. The size 10 in these fit perfectly right now without much room to spare. I kinda wish I got an 11 so he could possibly get two summers out of them. Id say they're true to size to 1/2 size small. Definitely size up if you're between sizes. He likes them and they're nicer and more comfortable than the average rubber thong.