I don't know how this item is measured, but I usually take a 7.5 or 8 so ordered an 8.  I can hardly get my foot in the slipper.  The nice shearling is going to be crushed in no time.  I couldn't even think about wearing a thin sock with them.  They also have a piece that comes about half way up your heel so - unlike a mule - there's no way for your heel to hang off the back for a bit more room.  I did not send them back because I purchased two different pairs at the same time and had worn the first pair for a month before I tried these, thus it was too late to return them.  The slippers are nice and well made but I would DEFINITELY order at least one if not two sizes beyond your regular size and something wider if you have anything other than a very narrow foot.