Every pair of glasses has a set of numbers printed on the inside of one arm, and those numbers tell you exactly how big the frame is. Once you know how to read them, you can shop for glasses online, compare frames across brands, and figure out why a pair fits well or poorly. The system is simple: three numbers, measured in millimeters, always listed in the same order.
Where to Find the Numbers
Pick up any pair of glasses and look at the inside of the temple arm (the part that goes over your ear). You’ll see a sequence of numbers, sometimes separated by dashes, spaces, or a small square symbol (□). That square is a standard optical industry marker that separates the first number from the second. A typical marking looks something like this: 52□18 140.
Those three numbers represent, in order: lens width, bridge width, and temple length. Every frame manufacturer worldwide follows this same sequence, standardized under an international system called ISO 8624. So whether you’re looking at a pair from a luxury brand or a drugstore shelf, the numbers mean the same thing.
Lens Width: The First Number
The first number is the lens width, sometimes called “eye size.” It measures the horizontal width of one lens at its widest point, in millimeters. This is the single most important number for determining how a frame fits your face.
Lens widths typically range from 31 mm to 60 mm. As a general guide based on the distance across your face from temple to temple:
- Narrow face (under 129 mm): lens width of 50 mm or smaller
- Medium face (130 to 139 mm): lens width of 51 to 55 mm
- Wide face (over 139 mm): lens width of 56 mm or larger
If your current glasses feel right, check the lens width and use it as your starting point when shopping for new frames. A difference of even 1 to 2 mm can change how a frame looks and feels on your face, so don’t assume a 54 mm and a 52 mm will be interchangeable.
Bridge Width: The Second Number
The second number, often separated from the first by that small square symbol, is the bridge width. This measures the gap between the two lenses, right where the frame sits on your nose. Common bridge sizes range from about 14 mm to 24 mm.
Getting this number right matters more than most people realize. A bridge that’s too narrow for your nose will pinch and leave red marks. A bridge that’s too wide lets the glasses slide down, so you’re constantly pushing them back up. If your current frames fit almost perfectly but tend to slip, try the same lens width with a bridge that’s 2 mm smaller. That small change is often enough to fix the problem.
Temple Length: The Third Number
The third number is the temple length, measuring from the front hinge all the way to the tip that curves behind your ear. The most common temple lengths are 135 mm, 140 mm, 145 mm, and 150 mm, so manufacturers tend to work in 5 mm increments here rather than offering a wide range.
Temples that are too short won’t reach comfortably behind your ears and the glasses will feel like they’re squeezing your head. Temples that are too long will stick out past your ears and the frames may feel loose or wobbly. When the length is right, the arms run horizontally along the sides of your head and rest evenly over each ear without bending outward or pressing inward.
The B-Measurement: Lens Height
Some frames list a fourth number, often labeled “B” on retailer websites. This is the vertical height of the lens, measured from top to bottom at its tallest point. You won’t always find it printed on the frame itself, but it’s listed in the specs when you shop online.
Lens height becomes especially important if you wear progressive lenses (no-line bifocals). Progressives pack three vision zones into a single lens, from distance at the top to reading at the bottom, and they need enough vertical space to work properly. For progressives, look for a B-measurement of at least 28 to 30 mm. Anything shorter and the reading zone at the bottom gets cramped, making it harder to see up close.
Pupillary Distance: The Number That’s Not on the Frame
There’s one more measurement that affects how well glasses work for you, and it’s about your face rather than the frame. Pupillary distance (PD) is the space between the centers of your pupils, measured in millimeters. The average adult PD is about 63 mm, though it can range anywhere from 50 mm to 70 mm.
Your PD determines where the optical center of each lens should sit. When the prescription’s focal point lines up with your pupil, you get the clearest, most comfortable vision. If it’s off, you may notice eyestrain, headaches, or slight distortion, especially with stronger prescriptions. Your eye care provider measures PD during an exam, and it should be on your prescription. If it isn’t, you can ask for it, and you’ll need it any time you order glasses online.
One useful detail: your PD actually gets about 3 mm narrower when you’re looking at something up close compared to looking into the distance. This is why some prescriptions list two PD values, one for distance and one for near vision.
How to Measure Your Face at Home
If you don’t have a pair of glasses to use as a reference, you can measure your face directly. You’ll need a millimeter ruler and a mirror, or a friend to help.
For face width, hold the ruler across your face at eyebrow level and measure from temple to temple. This gives you a rough idea of whether you’re in the narrow, medium, or wide category, which points you toward the right lens width range. For bridge width, measure across the top of your nose where glasses would sit. Compare that to the bridge measurements listed on frames you’re considering.
For temple length, measure from where the front of the frame would sit (roughly at your temple) to just behind your ear, following the horizontal line along the side of your head. Round to the nearest standard size: 135, 140, 145, or 150 mm.
Small Differences Matter More Than You’d Think
With clothing, a size medium from one brand is roughly a size medium from another. Glasses don’t work that way. A frame marked 52-18-140 and one marked 54-16-140 have the same temple length, but the 2 mm wider lens and 2 mm narrower bridge will make the second pair fit noticeably differently. The overall frame front will be wider, and it will sit differently on your nose.
Frame material also plays a role in how a given size actually feels. Metal frames, particularly those made from flexible alloys, can often be adjusted by an optician to fine-tune the fit after purchase. Acetate (plastic) frames can also be reshaped when heated. So if a frame is close but not perfect, an adjustment at an optical shop can sometimes bridge the gap. But starting with the right numbers gets you much closer to a comfortable fit from the start.
The most reliable approach is to find a pair you already love wearing, read the three numbers off the inside of the arm, and use those as your baseline. From there, you can experiment, knowing exactly what you’re changing and by how much.