I purchased these in a size Large (which is supposed to fit a 10.5-11.5) for my boyfriend who normally wears a size 10.5 ...and they slide right off his feet. I'd say go half a size down for a proper fit. He'll probably end up needing to replace this pair with a size Medium to have a comfortable fit. Other than that, I think ACORN makes the best slippers for men so I still rate this product pretty high.