The numbers printed inside your glasses frames aren’t your lens prescription. They’re the frame’s physical dimensions, measured in millimeters, and they tell you exactly how big the glasses are. You’ll typically find three numbers on the inside of the temple arm (the piece that hooks over your ear) or sometimes on the nose bridge. They follow a standard format like 52□18-140, and each one describes a different part of the frame.
What the Three Numbers Mean
Almost every pair of glasses uses the same three-number system. Here’s what each one tells you:
- First number (lens width): How wide each lens is at its widest horizontal point. For adults, this typically ranges from 40 mm to 60 mm. A smaller number means a narrower lens opening, which generally suits a narrower face.
- Second number (bridge width): The distance between the two lenses at their closest point, which is the part that sits over your nose. This ranges from 14 mm to 24 mm. It’s not the width of the physical bridge piece itself, but the gap between the inner edges of the lenses.
- Third number (temple length): How long the arms are from the hinge to the curved tip that wraps behind your ear. This usually falls between 120 mm and 150 mm.
The small square symbol (□) between the first and second numbers indicates the manufacturer uses the standardized “boxing system” for measurements. So 52□18-140 means: 52 mm lens width, 18 mm bridge width, 140 mm temple length. Sometimes the square is replaced by a dash or slash, but the order stays the same.
Other Numbers and Codes on the Frame
You’ll often see more than just those three numbers. The inside of the temple arm can look cluttered with text, and it helps to know what’s sizing information and what isn’t. Model numbers identify the specific frame style and are usually a longer alphanumeric code. Color codes are short abbreviations that tell the manufacturer which finish or colorway the frame is. Some frames also carry a CE mark, which means they meet European safety standards. None of these affect fit.
The easiest way to pick out the sizing numbers is to look for the three-number sequence separated by symbols or dashes. If you see something like “RB5154 52□18-140,” the “RB5154” is the model number and the three numbers after it are your frame dimensions.
The Measurement You Won’t Find Printed
There’s a fourth dimension that matters for fit but rarely appears on the frame: lens height, sometimes called the “B measurement.” This is the vertical depth of the lens at its tallest point. It’s especially important if you wear progressive lenses (no-line bifocals), because those lenses need enough vertical space to fit the different vision zones from distance at the top to reading at the bottom.
If the lens height is too shallow for a progressive design, the reading portion at the bottom gets cut off, making the glasses uncomfortable or useless for close-up tasks. Most online retailers list the B measurement on the product page even though it’s not stamped on the frame itself. If you’re shopping for progressives, look for a lens height of at least 28 to 30 mm, though your optician can recommend a minimum based on the specific lens design.
How Bridge Width Affects Fit
Of the three printed numbers, bridge width has the biggest impact on comfort. A bridge that’s too wide lets the frame slide down your nose, and gravity pulls it to the widest part of the bridge where it sits even more loosely. A bridge that’s too narrow pinches or perches the glasses too high on your face.
People with higher nose bridges often do best with a snug (but not tight) bridge measurement and frames with a keyhole-shaped bridge cutout, which contours around the nose rather than sitting flat against it. People with a lower or flatter nose bridge typically need a wider bridge number or frames with built-in nose pads that can be adjusted. If your current glasses fit well, the bridge number printed inside them is the single most useful reference point when shopping for a new pair.
How to Measure When the Numbers Are Worn Off
If the markings on your frames have rubbed away, you can measure them yourself with a millimeter ruler. For lens width, measure the widest horizontal distance across one lens opening, from the inner edge of the frame rim on one side to the inner edge on the other. For bridge width, measure the shortest distance between the two lenses at the point where they’re closest together, even if that point is tucked deep inside the frame near the nose pads rather than at the visible bridge.
Temple length is harder to measure accurately because you need to include the curved portion. Lay the arm flat against a ruler from the hinge to the very tip. If the tip curves, try to follow the curve as closely as you can. Being off by a couple of millimeters on temple length matters less than being off on bridge or lens width, since the temples can usually be adjusted by an optician.
Using Frame Numbers When Buying New Glasses
These three numbers give you a reliable starting point when ordering glasses online or trying frames in a store. Write down the numbers from a pair that already fits you well, then compare them to the specs listed on any new pair you’re considering. Staying within 1 to 2 mm on lens width and bridge width will generally give you a similar fit. Temple length can vary a bit more without causing problems, but jumping from 135 mm to 150 mm would noticeably change how the arms feel behind your ears.
Keep in mind that two frames with identical numbers can still feel different depending on the frame shape, material, and weight. A round frame and a rectangular frame with the same 52 mm lens width will sit differently on your face because the lens height and overall frame coverage are different. The numbers get you in the right range; trying the frames on (or checking the return policy) gets you the right fit.