What do the numbers on glasses mean? Reading the size inside the arm

Turn your glasses over and look inside one arm. Most frames, prescription or sunglasses, carry three small numbers there, something like 52□18 140. They are the size of your glasses, in millimetres. Once you can read them, you can buy a new frame, a case or a replacement pair without guessing.

The three numbers, one by one

The small square between the first two numbers means the frame was measured with the boxing system, the standard way opticians size a lens: as if each lens sat inside a box. Some brands print a dash instead (52-18-140). It means the same.

Small, medium or large?

Lens width is the number most people mean when they talk about frame size:

• Small: lens width up to about 48 mm. Often round and vintage frames, and frames for narrow faces.
• Medium: about 49–53 mm. Most everyday glasses sit here.
• Large: about 54 mm and above. Many aviators and statement sunglasses.

If a pair you own fits you well, its numbers are the best guide to the next one. A difference of 1–2 mm in lens width is hard to notice. Five is not.

From the numbers to the width of the frame

The numbers do not give the full width of the glasses directly, but they get you close. Add the two lenses and the bridge, then about 10 mm for the rims and hinges:

For a precise figure, use a ruler: measure across the front of the glasses, from the outer edge of one hinge to the other.

The other markings

Next to the size you will often find more letters and numbers. They are not measurements:

• Model number and colour code, for example 5021 C2. Useful when ordering a replacement.
• CE: the frame meets European safety requirements.
• UV400 on sunglasses: the lenses block ultraviolet light up to 400 nanometres.
• Cat. 0 to 4 on sunglasses: the filter category, from almost clear (0) to very dark (4). Category 3 is the usual for bright sun. Category 4 is too dark for driving.

What the numbers tell you about a case

A case is chosen by the width of the frame and the height of the lens. Our case is made for frames up to about 148 mm (5.8 in) across, with lenses up to about 50 mm (2 in) tall. In the table above, the first two fit comfortably and the oversized pair sits right at the limit.

That covers most everyday glasses and sunglasses: classic square frames, round, cat-eye, browline, readers and most aviators. It is not made for wraparound sport frames, rimless shields or oversized frames with very thick arms. Your glasses slip in through the opening at the back, arms folded outside.

Lens height is the one number that is never printed. If your lenses are unusually tall, measure one from top to bottom before you order.

Check yours in a few seconds

Our fit checker does the sum for you: type in the first two numbers from the arm. If you are still not sure, send a photo of your glasses to support@sonnaer.com and we will tell you honestly.

Questions people ask

Where are the numbers on glasses?

Inside one of the arms, usually the left, near the hinge. On some frames they are on the bridge or on the inside of the front. On older glasses they may have worn away.

Is the first number the lens width?

Yes. The first number is the width of one lens in millimetres, the second is the bridge, and the third is the length of the arm.

What if my glasses have no numbers?

Measure them. Use a ruler for the width of the front and the height of one lens. For a new prescription frame, your optician can also measure the old one.

Do sunglasses use the same numbers?

Yes. Sunglasses are sized the same way, though some fashion brands leave the size off. Look for the model number instead and search for its size.

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