You will then see a product page showing photos and a description of the selected clip-on.
Scroll down that page and click the link called: "View Size Guide".
Measure the dimensions of your glasses in millimetres:
lens width, including the rims, measured half way up the lens
total width across the frame, measured half way up the lens, from the outer rim of the left lens to the outer rim of the right lens
height of a lens, including the rims, measured half way across the lens
Compare these measurements with those shown on the Size Guide.
Do not use the size guide as a printed template because it will print out at a slightly different size depending on which computer operating system you use
For rimmed clip-ons, the dimensions are given for the size of the tinted lens area, excluding the rims
For rimless clip-ons, the actual tinted lens area is as the dimensions shown plus 1 mm all round (equivalent to the rim width on the rimmed models)
Select the size of clip-on sunglasses that corresponds most closely with your frames. If in doubt, buy two sizes and return the one that does not fit for a refund.