I typically wear a size 8-1/2, sometimes a 9 in athletic shoes, so I ordered a size 9. They were a little big, but fit okay with thick socks. It's possible I could have fit a size 8, especially if I only planned on wearing them with thin socks, but I was too lazy to return them. I have wide feet, but these boots were very roomy, even with thick socks. I was surprised how comfortable they were - no wonder my 5 year old loves her Crocs rainboots.