نظارة شمسية مربعة SL364MASK من سان لوران

Every frame carries three measurements in millimetres, printed in small type on the inside of one temple arm. They read like this:

63□15 145

63

Lens width. The width of a single lens at its widest point.

15

Bridge. The gap between the two lenses, the part that sits on your nose.

145

Temple length. The arm that runs from the hinge to behind your ear.

Finding them on a pair you own

Open the arms of the glasses you wear now and turn the left one so the inside faces you. The three numbers run along it, usually near the hinge, separated by a small square or by dashes. The model code and the colour code often sit beside them. If the left arm is bare, check the right arm and the inside of the bridge.

Match those numbers against the lens width listed in the specification on this page and you will know how this frame compares to one you already like the fit of. A difference of one or two millimetres is not something you will feel. Five or more is.

Or let an optician read it

Photograph the inside of your current temple arm and send it to us on WhatsApp. Our opticians will read the size off the picture and tell you whether the frame you are looking at will sit correctly on your face, before you order.

Send a photo on WhatsApp Read the full guide to frame numbers