I wear a size 8 in flats and 7 1/2 in high heels due to my narrow heels.  The medium fits my foot perfectly with little room to spare.  The first 2 days I wore them I thought I should have bought the LARGE, but now on my third day, they have stretched out and formed to my feet and the MEDIUM is ideal.  They are lightweight, soft and perfect for wearing around the house and yard.  I'll continue to purchase these.