Ditto for Denise's advice. My first pair of Sidis were 36s since I wear a size 6 or even 5.5 in street shoes. After I started riding longer distances, I had a terrible foot swelling experience on a 40 mile ride in hot weather. I bought my Dominators in a size 38 and they are perfect. My mtb shoes, also Sidis, but not Dominators are 37.5 and are on the edge of too tight. Surly Girl graciously gave me a pair of mtb shoes in another brand that didn't fit her. They are 37s and are almost too big. I will wear them for cold weather riding on the mtb, with wool socks and use my Sidis for spin class. That way, I am not bringing mud into the gym.