My feet are a size 12, but I order size 13 in shoes, both tennis and office, because I like to wear cushioned socks. For these Sockwa sand socks, I ordered a 12, and they fit perfectly. They do not slide around on my feet and they keep the sand out very well. When the sun is out, we play on some incredibly hot sand for weekend pickup ball, and these socks are the bomb. My feet still feel great after three hours of sand volleyball.