How to know your glasses size? Put a card on the bridge of your nose and under your eyebrows. If the edge of the card goes outside the edge of your eye, then you are a size S. (Small size), therefore, the gauge of your glasses has to be less than 50 mm. If the edge of the card ends on the outside of the edge of your eye, then you are a size M. (Medium size), therefore, the size of your glasses has to be between 51mm and 54mm. If the edge of the card ends before the outside of your eye, then you are a size L. (Large size), more than 54mm, therefore, the size of your glasses must be more than 54mm. You can check the caliber of each of the Kayros glasses in the "Details" section.