I'm a true size 9, but these straps were tight & left marks on my feet, so I ordered the 10 and they are much more comfortable. The 10s are also just a little bigger around my heel and toes, whereas the 9 was too exact to the size of my foot - I prefer to have a little extra material to protect my bare foot, than it be just at the edge of my feet. So the 10s are now mine.