The Crocs Duet Athens were marked women's size 6.  I wear a 6 1/2 shoe; a 6 sandal.  The flip flop was about a 5 or 5 1/2.  It was about 1/2 size too small. I returned them, called the company and reported that the sizing for this thong was two to 2 1/2 sizes smaller than the Crocs Athens