Choose the rectangle style that most closely resembles that of your prescription glasses.
Use a millimeter (mm) ruler to measure across the front of one of your prescription lenses (see example diagram to the right)—this will be your size for your clip-ons. Make sure that the clip-on size you select is at least as large as your lens measurement. If your lens measurement is not listed in the size options, select the next largest size clip-ons. For example: if your lens measures 47mm, your best option would be a size 48.