Choose the square 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.