The numbers printed on your eyeglass frames are size measurements in millimeters. You’ll find up to three of them, always listed in the same order: lens width, bridge width, and temple length. They’re usually printed on the inside of one temple arm (the piece that goes over your ear), sometimes separated by dashes or a small square symbol. Once you know what each number means, you can compare frames and shop for glasses online with confidence.
The Three Core Numbers
Pick up your glasses and look at the inside of the temple arm. You’ll see a string of text that might look something like “52-18-140.” Those three numbers tell you everything about the frame’s dimensions.
The first number is the lens width, sometimes called “eye size.” It measures the width of each lens at its widest point, not including the surrounding frame material. For adult glasses, this typically falls between 31 mm and 60 mm. A smaller number means a narrower lens, which generally suits a narrower face.
The second number is the bridge width, the distance across the small piece that rests on your nose between the two lenses. Most glasses fall between 14 mm and 24 mm here, with the majority clustering in the 16 to 21 mm range. This is the number that determines whether your glasses sit comfortably on your nose or constantly slide down.
The third number is the temple length, measured from the hinge screw at the front of the frame all the way to the tip that curls behind your ear. This is always a three-digit number, typically between 115 mm and 155 mm.
How to Read the Full String
The sizing numbers share space on the temple arm with other codes from the manufacturer, which can make things confusing. A Ray-Ban frame, for example, might read “RB3447 001/51 50-21 145 3N.” In that string, “RB3447” is the model number, “001/51” is a color code identifying the specific frame and lens tint, and “50-21 145” are the three size measurements: 50 mm lens width, 21 mm bridge, 145 mm temple length.
Oakley uses a similar format but separates the numbers with a small square symbol instead of dashes, so you might see “60◻14 132.” Lindberg lists a color code first, then the measurements: “P10 49-20-140.” The pattern is always the same regardless of brand. Two-digit number, two-digit number, three-digit number, in that order. If you spot that pattern, you’ve found the sizing.
Why Bridge Width Matters Most for Comfort
Of the three measurements, bridge width has the biggest impact on how your glasses feel day to day. A bridge that’s too narrow will pinch the sides of your nose and leave red marks. One that’s too wide lets the frames slide forward every time you look down. If you have a pair that fits well, check the bridge number and use it as your baseline when shopping for new frames.
People with a wider nose bridge generally need 18 mm or above, while those with a narrower bridge do better around 16 to 17 mm. A difference of just 1 to 2 mm is noticeable, so treat this number seriously when comparing frames online.
Getting Temple Length Right
Temple length controls where the curved end of the arm hooks behind your ear. The standard ranges break down like this:
- 120 to 129 mm: short
- 130 to 139 mm: medium
- 140 to 150 mm: long
- 150 mm and above: extra long
When temples are too short, the curve digs into the area behind your ear instead of resting gently over it, creating a pinching pressure that builds into a headache over hours. When they’re too long, the curve starts too far back to hook onto your ear, and the frames slide down your nose constantly. Most adults land comfortably at 140 mm, but if you’ve ever had either of those problems, checking this number on your current pair will tell you which direction to adjust.
Lens Width vs. Total Frame Width
A common mistake when shopping online is assuming the lens width tells you how wide the frames will look on your face. It doesn’t, at least not on its own. The total frame width depends on lens width, bridge width, and the thickness of the frame material around each lens. Two frames with a 52 mm lens width can look quite different if one has thick acetate rims and the other is thin wire.
Some retailers list a total frame width measurement separately, but this number won’t appear on the frame itself. To estimate it, you can roughly add: lens width times two, plus bridge width, plus a few millimeters for the frame edges on each side.
The Fourth Number You Might Not See
There’s a measurement called the “B measurement” or lens height that doesn’t usually appear printed on the frame but matters if you’re ordering progressive or bifocal lenses. It’s the vertical height of the lens at its tallest point. Progressive lenses need enough vertical space to fit distinct viewing zones for distance, intermediate, and reading vision. If the lens height is too short, those zones get compressed and the usable area for each shrinks. Your optician will measure this when fitting you, but if you’re buying frames online for progressives, look for frames with a lens height of at least 28 to 30 mm.
Safety Markings on Protective Eyewear
If you’re looking at safety glasses or protective eyewear rather than everyday frames, you’ll see a different set of markings. The most important is “Z87,” which means the eyewear meets the current American National Standards Institute impact protection standard. A “+” after Z87 means the frames are rated for high-velocity impact. Other letter codes indicate specialized protection: “D3” for splash and droplet resistance, “D4” for dust, “X” for anti-fog coating, and “W” followed by a number for welding shade level. These markings appear on both the lenses and the frame and are separate from any sizing numbers.