The great thing about these is they're included in Prime Wardrobe so you can order two sizes to find the right size for you. Ive been reading to size up, but wanted to share my story. I wear a 5.5-6 and have narrow feet. The 6 was too big, looked okay but was terribly uncomfortable. The 5 looked very snug (no extra space anywhere) but was a perfect fit comfort wise. These are jelly and have hard grooves to fit into the contour of your feet, so to be safe order two sizes and try on.