The numbers printed on your glasses represent three frame dimensions, all in millimeters: lens width, bridge width, and temple length. You’ll usually find them stamped on the inside of one temple arm (the piece that runs from the hinge to behind your ear), and they always appear in that same order. A frame marked 52-18-140 has 52mm lenses, an 18mm bridge, and 140mm temples.
Lens Width: The First Number
The first number is the horizontal width of each lens, sometimes called “eye size.” It’s the single biggest factor in how wide the glasses look on your face. Frames with a lens width of 50mm or under are considered narrow, 51mm to 55mm is medium, and 56mm or above is wide. Those categories roughly correspond to total frame widths of under 129mm, 130 to 139mm, and over 139mm from temple to temple.
Lens width matters most when you’re buying glasses online and can’t try them on. If your current pair fits well, matching this number within a millimeter or two is the fastest way to get a similar fit. A jump of even 3 to 4mm can make frames feel noticeably larger or smaller on your face.
Bridge Width: The Second Number
The second number measures the shortest gap between the two lenses, right where the frame crosses your nose. This is the measurement that controls how high or low your glasses sit.
A smaller bridge width (around 14 to 18mm) pulls the frame closer to your face and positions it higher on your nose, which works well if you have a narrow nose bridge. A wider bridge (18mm and above) lets the frame sit a bit lower and accommodates a broader nose. If your current glasses slide down constantly, you likely need a narrower bridge. If they pinch or sit too high, a wider one will help.
Frame thickness plays a role here too. Thicker, heavier frames need slightly wider bridges to distribute their weight comfortably. So if you normally wear a 17mm bridge in thin metal frames but switch to chunky acetate, sizing up to 18 or 19mm can prevent pressure marks on the sides of your nose.
Temple Length: The Third Number
The third number is the length of each arm, measured from the hinge screw all the way to the curved tip that hooks behind your ear. Common sizes are 135mm, 140mm, and 145mm, though they range from about 120mm to 150mm.
Temples that are too short won’t reach far enough behind your ears, and the glasses will feel like they’re slipping forward. Temples that are too long will bow out awkwardly or need to be bent at a sharper angle. Most adults land between 135mm and 145mm, and a difference of 5mm is usually noticeable in comfort but easy to adjust if an optician bends the tips for you.
Lens Height: The Number That’s Not Printed
One measurement you won’t find stamped on the frame is the lens height, sometimes called the B-measurement. This is the vertical distance from the top of the lens opening to the bottom. It matters most if you wear progressive (no-line bifocal) lenses, because those lenses pack three vision zones into that vertical space: distance at the top, computer range in the middle, and reading at the bottom.
Standard progressive lenses need a minimum lens height of about 28mm to fit all three zones comfortably. Specialized “short corridor” designs can squeeze into frames as shallow as 22mm, but the reading area at the bottom gets noticeably cramped. If you’re picking frames for progressives, aim for at least 28 to 32mm of vertical height. For dedicated computer or office lenses, 30mm or more is ideal because the intermediate zone benefits from extra room.
Total Frame Width
You might expect to calculate total frame width by adding lens width plus bridge width plus lens width (so 52 + 18 + 52 = 122mm for a 52-18 frame). That gives you the internal span of the front piece, but the actual total width is slightly larger because it includes the hinges and any design features on the sides. The only reliable way to get this number is to lay a ruler across the entire front of the frame from the widest point on one side to the other.
This total width is what determines whether a frame matches the width of your face. Ideally, the edges of the frame should line up roughly with your temples. Frames significantly wider than your face look oversized, while frames narrower than your face create pressure on the sides of your head.
How to Measure if the Numbers Are Gone
On older frames, the printed numbers sometimes wear off completely. You can recover them with a millimeter ruler or, even better, a small clear ruler called a PD ruler (available at most optical shops for a few dollars).
For lens width, measure the widest horizontal point inside the lens opening. For the bridge, measure the shortest distance between the two lenses at the point where they’re closest together, even if that spot sits deeper inside the frame than the visible bridge piece. For temple length, measure from the hinge screw to the very end of the curved tip.
One small catch: if you’re measuring with the lenses still in the frame, the rim covers about a millimeter of the lens on each side. Add 1mm to both your lens width and lens height measurements to compensate, and subtract 1mm from your bridge width. Those tiny corrections keep your numbers accurate enough to order a well-fitting frame online.
What Opticians Use Beyond the Three Numbers
The three printed numbers are designed for consumers, but optical labs rely on a few additional measurements when cutting your lenses. The most important is the effective diameter, which is the longest line you can draw across the lens opening, doubled. Labs use this to choose the smallest uncut lens blank that will still fill your frame after centering the lens on your pupil. If the blank is too small, the finished lens won’t cover the entire opening, a problem called “cutout.” You’ll never need to calculate this yourself, but it explains why certain frame shapes (tall, round, or cat-eye) sometimes cost more to fill: they require larger blanks.
Your pupillary distance, the space between the centers of your pupils, is another measurement the lab pairs with your frame numbers. It tells them exactly where to position the optical center of each lens so you’re looking through the sharpest part. This one isn’t printed on the frame because it’s a measurement of your face, not the glasses. Your eye care provider measures it during an exam, or you can find it on your prescription.