I'm not certain, but when I bought mine, I was measured. The box sticker says 36 1/3 EUR, US 4.5, UK 3.5, MM 239 (So, I haven't a clue what my actual size is....) She also made me walk around the room to see how or if I pronated my feet when I walked. Inquired about whether I had any dizziness issues - not sure if that had anything to do with the choice of shoe style or not. At least, I was impressed - since I was spending the grocery $$$$ - I wanted a good fit. The store also has a 14 day trial - so I wore them around the house for the first few days before venturing outside (didn't want to skuff them up - just in case.)