The numbers printed on your glasses follow a standard format: lens width, bridge width, and temple length, always in that order, always in millimeters. You’ll usually find them printed on the inside of one temple (the arm that goes over your ear) or along the bridge between the lenses. Once you know the pattern, you can read any pair of glasses in seconds.
The Three Numbers and What They Mean
Every pair of glasses carries up to three numbers that describe its physical dimensions. They’re sometimes separated by dashes, sometimes by a small square symbol (□), and sometimes just by spacing. Here’s what each one tells you:
- First number: lens width. This is the horizontal width of one lens, measured in millimeters. It only covers the clear lens you look through, not the surrounding frame material. Most glasses fall between 40 and 60 mm.
- Second number: bridge width. This is the shortest distance between the two lenses. It corresponds to how the frame sits on your nose. Typical range is 14 to 24 mm.
- Third number: temple length. This measures the full length of each arm, from the hinge screw all the way to the curved tip behind your ear. Common lengths are 135, 140, 145, and 150 mm.
So if you see “52-18-140” on your frames, that means 52 mm lenses, an 18 mm bridge, and 140 mm temples.
Where to Find the Markings
The most common location is the inside of the left temple arm. Look near the hinge or about halfway down the arm. Some frames print the lens width and bridge width on the bridge itself (the part that crosses your nose) and put the temple length on the inside of the arm. If you see a small square symbol between two numbers, like 48□18, that’s the lens width and bridge width using the industry-standard “box system” measurement format.
On some minimalist or rimless frames, the markings may be tiny or placed on the nose pad arm. If the numbers have worn off, you can measure the frame yourself with a millimeter ruler, though an optician can do it more precisely.
Telling Size Numbers Apart From Model and Color Codes
Most frames carry more than just three size numbers. You’ll also see model numbers, color codes, and brand names, which can make the whole string look confusing. The trick is knowing what format each type follows.
Take a Ray-Ban as an example: “RB3447 001/51 50-21-145 3N.” Here, RB3447 is the model number, 001/51 is the color code for the frame and lens tint, and 50-21-145 are the three size measurements. Oakley uses a similar pattern: “OO4141-01 60□14 132” gives you the model (OO4141), color variant (01), and sizes (60, 14, 132).
The size numbers are almost always two or three digits, grouped together, and separated by dashes or a square symbol. Model numbers tend to be longer, often with a letter prefix. Color codes frequently include slashes or letters. Once you spot the cluster of three short numbers, you’ve found your frame dimensions.
Why These Numbers Matter for Fit
Getting the right frame size isn’t just about aesthetics. Each measurement directly affects comfort and how well you can see through your lenses.
Lens width determines how much of your field of vision the lens covers and how the frame looks on your face. Too wide and the frames overwhelm smaller faces; too narrow and you may notice the frame edges in your peripheral vision. If you’re ordering new glasses online, staying within 2 mm of your current lens width is a safe bet.
Bridge width is the single biggest factor in whether glasses feel comfortable. The bridge rests directly on your nose and supports most of the frame’s weight. A bridge that’s too narrow can pinch, leaving red marks or even causing headaches. A bridge that’s too wide lets the glasses slide down constantly. If your frames shift upward when you smile, the bridge likely doesn’t match your nose shape. Even a 2 mm difference in bridge width can change how glasses feel over a full day of wear.
Temple length affects how securely the frames stay on your head. Temples that are too short won’t hook properly behind your ears and the glasses will feel loose. Temples that are too long will stick out past your ears or bow outward. Because temple lengths come in 5 mm increments (135, 140, 145, 150), most people can find a close fit, but the right length matters especially if you wear glasses during physical activity.
How to Use Your Numbers When Shopping
If you already own a pair of glasses that fits well, write down all three measurements before shopping for a new pair. You don’t need to match them exactly, but staying close helps. A difference of 1 to 2 mm in lens width or bridge width is usually unnoticeable. Temple length can vary by about 5 mm without creating problems for most people.
When shopping online, filter by frame size if the retailer offers that option. If not, check the product details page, where the three measurements are almost always listed. Compare them to your current pair rather than guessing based on terms like “small” or “large,” which vary between brands.
If you’re buying your first pair and don’t have a reference frame, an optician can measure your face directly. The key measurements they’ll take are your nose bridge width and the distance between your pupils, which helps determine what lens width will center your eyes properly in the frame. You can also try frames in a store, note the three numbers from the pair that feels best, and use those as your baseline for future purchases.