Every pair of glasses has three key numbers printed somewhere on the frame, usually on the inside of the temple arm. These numbers, always in millimeters, tell you the lens width, bridge width, and temple length. Once you understand what they mean and how to measure them yourself, buying glasses online or comparing frames becomes straightforward.
What the Three Numbers Mean
Pick up any pair of glasses and look along the inside of one of the arms or on the bridge. You’ll find a sequence of numbers that looks something like 52-18-140. Each number describes a different part of the frame:
- First number (lens width): The horizontal width of one lens at its widest point, sometimes called “eye size.” In this example, 52mm.
- Second number (bridge width): The distance between the two lenses, right where the frame sits on your nose. Here, 18mm.
- Third number (temple length): The full length of the arm that runs from the front of the frame back over your ear. In this case, 140mm.
Some frames also list a fourth number for the lens height, which matters most for progressive or bifocal lenses. But those three core measurements are what determine fit.
How Lens Width Affects Fit
Lens width is the single most important number for getting the right frame size. It controls how wide the glasses sit across your face and how your eyes are positioned behind the lenses. As a general guide:
- Small: 47 to 49mm
- Medium: 49 to 53mm
- Large: 53 to 55mm
If you already own a pair of glasses that fits well, the lens width printed on those frames is your best starting point. When shopping for a new pair, staying within 1 to 2mm of that number keeps the fit consistent. Going much wider will make the frame look oversized, and your eyes may not align with the optical center of the lenses, which can affect your vision.
How to Measure an Existing Pair
If your current glasses don’t have legible numbers (they can wear off over time), you can measure the frame yourself with a millimeter ruler.
For lens width, measure the widest horizontal point of one lens from the inner edge of the frame to the outer edge. Don’t include the frame material itself, just the opening where the lens sits. For bridge width, measure the gap between the inner edges of the two lenses, right across the nose area. Both measurements should be taken in a straight horizontal line.
Temple length is trickier because the arms curve. Lay the arm flat and measure from the hinge (where it connects to the front of the frame) all the way to the tip that hooks behind your ear. Most adult temples fall between 135 and 150mm, with 140mm being the most common size.
How to Measure Your Face Directly
If you don’t have a well-fitting pair to reference, you can measure your face to figure out what frame width you need. Pull your hair back from your ears and hold a millimeter ruler (or flexible tape measure) horizontally just above your eyebrows. Measure from the outer edge of one temple to the outer edge of the other, where the arms of glasses would normally sit. Keep the ruler level and straight.
This measurement gives you your total frame width. It’s not the same as lens width, but it tells you how wide the entire front of the frame should be. Most online retailers list total frame width alongside the three standard numbers, making it easy to compare. If a retailer only lists lens and bridge width, you can approximate total frame width by adding both lens widths plus the bridge width plus a few millimeters for the frame material on each side.
Choosing the Right Bridge Width
Bridge width typically ranges from about 14mm to 24mm, and getting it right determines whether your glasses sit comfortably on your nose or slide down constantly. A bridge that’s too narrow will pinch. One that’s too wide will let the frame slip.
Your nose shape matters here as much as raw measurement. People with a high nose bridge (a noticeable bump or ridge between the eyes) can usually wear standard frames without issue. If you have a low nose bridge, where the area between your eyes is relatively flat or level with your cheekbones, glasses tend to slide down and rest on your cheeks. In that case, look for frames specifically designed for a low bridge fit, or choose a wider bridge measurement to keep the frame from sitting too high or pinching.
To figure out your bridge width without a reference pair, measure the width of your nose at the spot where glasses normally rest, right between your eyes. That gives you a rough target, though you may need to go 1 to 2mm wider for comfort.
Getting Temple Length Right
Temple length is the measurement people overlook most often, but it’s what determines whether your glasses stay in place or feel like they’re squeezing your head. The arms should reach just past your ears without extending far beyond them.
Short temples (around 135mm) work well for smaller faces and petite frames. Standard temples (140 to 145mm) fit most adults comfortably. Long temples (150mm or more) suit larger faces or anyone who prefers a looser fit. If temples are too short, they’ll press uncomfortably behind your ears. Too long, and the glasses will feel loose and slide forward when you look down.
You can measure temple length on your face by holding a ruler from the front of your ear (where a hinge would sit at your temple) to just past the top of your ear where the arm would curve down. Add about 40 to 50mm to account for the portion of the arm that wraps behind the ear, and you’ll have a reasonable target length.
Putting It All Together
The fastest way to find your size is to start with a pair that already fits. Check the inside of the temple arm for those three numbers and use them as your baseline. If you’re between sizes or trying a different frame shape, lens width is the number to prioritize. Bridge and temple measurements have some flexibility since nose pads can be adjusted and temple tips can be bent by an optician, but lens width is fixed once the frame is made.
When ordering online, pay attention to whether the retailer lists measurements in the standard three-number format or uses their own sizing system (small, medium, large). If they use general sizes, check the detailed specs to see the actual millimeter values. A “medium” from one brand can easily be a “large” from another. The numbers are the only reliable way to compare across brands.