Step 01
Find the brand or model name
First, look for a logo or brand name somewhere on the frame. Many sunglasses also carry a separate model name, usually printed on the inside of the temple arm. For instance, Ray-Ban is the brand, while Wayfarer is the model.
Step 02
Find the model number
The model number is usually stamped on the inside of the temple arm or across the bridge. It typically opens with a couple of letters — 'RB' for Ray-Ban, 'OO' for Oakley — followed by a run of digits.
Step 03
Work out your lens width
That run of digits usually covers lens width, bridge width, and temple length in one string. For example, '54□16 140' breaks down to a 54mm lens, 16mm bridge, and 140mm temple. It's the first number, the lens width, that matters most for a replacement lens order. Brands like Persol, Prada, and Ray-Ban often reuse a model number across several different sizes, so don't assume — check this figure carefully.