Understanding frame sizes

The three numbers

Look inside one of the arms of your glasses. You will usually see something like 52 □ 18 145 or 52-18-145, alongside a model name and color code. All three numbers are in millimeters.

Across our collection, adult frames run from about 49 to 62 mm in lens width, and kids’ frames from about 39 to 49 mm.

The fourth number: lens height

Lens height, often called the B measurement, is the vertical depth of the lens opening. It is rarely printed on the frame, but it matters a great deal for progressive and bifocal lenses. Those lenses stack distance, intermediate and reading zones from top to bottom, so they need enough vertical room. A very shallow frame can squeeze the reading area. The optician will check the height of the frame and measure where your pupil sits in it before ordering multifocal lenses.

How to use your current frame as a guide

If your current glasses feel good, they are the best size reference you have.

1. Find the numbers inside the arm and write them down.
2. Note what you like about the fit: how the bridge sits, whether the arms feel right.
3. Note what bothers you, if anything: sliding, pinching at the temples, marks on your nose.
4. Measure the overall front width with a ruler, hinge to hinge across the front. The three numbers do not capture this directly, and it is a useful extra check.

When you shop, look for frames within a few millimeters of your current lens width and bridge, then adjust based on what you noted.

A worked example

Say your current frame reads 52 □ 18 145 and fits well, except that it slides down your nose.

• Lens width. Frames from about 50 to 54 mm will likely feel similar in width. Going much larger changes the look and can move your eyes off the lens center.
• Bridge. Sliding often means the bridge is a little wide for your nose. A frame with a 17 mm bridge, or one with adjustable nose pads, may sit more securely.
• Temple. Arms of 140 to 145 mm will likely suit you. If they feel loose behind the ears, slightly shorter arms or a temple adjustment can help.
• Overall width. To compare widths roughly, add both lens widths and the bridge: 52 + 52 + 18 = 122 mm across the lenses. A 54 □ 16 frame comes to 124 mm, so it is about 2 mm wider across the lenses even though its bridge is narrower. Frame fronts extend past the lenses, so treat this as a rough guide and measure the real front if you can.
• If you want progressives, check that the new frame has enough lens height. A frame much shallower than your current one may leave less room for the reading zone.

Now compare two candidate frames: a 51 □ 17 140 and a 55 □ 19 145. The first is close to your current size with a slightly narrower bridge, which may fix the sliding. The second is wider overall and has a wider bridge, so it would probably slide more. The first is the better place to start.

Why the numbers are only a starting point

Two frames with identical numbers can fit differently. Shape changes how width feels: a round 52 and a rectangular 52 sit differently on the face. Bridge style matters: a fixed acetate bridge and adjustable nose pads with the same measurement can rest at different heights. The curve of the front, the thickness of the material, and where the hinges sit all play a part.

That is why an optician checks the fit in person and adjusts the frame to your face. The numbers narrow down the choices; try-on and adjustment finish the job.

Sizes for kids

Children’s frames follow the same three-number system with smaller values. Kids grow, so a frame that fits well now, with arms that hold securely behind the ears, is better than one bought “to grow into.” A loose frame slides down, and children then look over the top of their lenses instead of through them.

Next step

Write down the numbers from your current pair and bring the glasses with you. Browse frames and collections, then visit so an optician can confirm the fit and take your lens measurements.