Every frame has its fit printed on the arm, and almost nobody reads it. Learn those three numbers and the shape rules stop being guesswork - you can filter an entire website down to frames that will actually sit right before you look at a single photo.
They read like 52-18-145: lens width, bridge width and arm length, all in millimetres. Lens width plus bridge should land close to your own face width; a bridge number too small pinches and too large slides. Once you know a frame that fits you, those numbers are the search filter you carry between brands.
The old rule - contrast the face shape - is roughly right: angular frames give a round face definition, rounded frames soften a square jaw, and a frame wider than your cheekbones adds width a long face wants. The more useful rule is that the top line of the frame should follow your brow line, and the frame should not sit on your cheeks when you smile.
A strong short-sighted prescription is thinner in a smaller, rounder lens, so large square frames fight it. A strong long-sighted prescription magnifies the eye, and a heavier acetate frame carries that better than a thin metal rim. A low nose bridge slides in most standard frames and is exactly what adjustable nose pads and Asian-fit bridges exist for.
How do I measure my face width?
Temple to temple in millimetres, in front of a mirror with a ruler. Compare it to lens width plus bridge plus about six millimetres per rim and you can filter accurately online.
Why do my glasses slide down?
Usually the bridge is too wide for your nose or the bridge sits too low for its shape. Nose pads or a smaller bridge number fix it; tightening the arms does not.
Can it advise on my prescription?
No. Prescriptions and eye health are for an optometrist. This is about how a frame fits and looks.
Is it free?
Yes, and there is no signup.