“I have narrow medium volume feet. I wear a 39 Birkenstock and 8.5 running shoe. Bought Slipper in a 39. Really nicely made and cozy but a little loose around the top of the foot…nothing I can’t handle as long as I don’t try to run after my pup! The haflingers I also like are a bit more supportive in the arch but these are great. Took longer than the “2-4 Day Delivery” so plan accordingly. The 40 was way too big.”