The numbers printed inside your glasses frames are size measurements in millimeters. They always appear in the same order: lens width, bridge width, and temple length. Once you know what each one means, you can use them to find frames that actually fit your face when shopping online or replacing a favorite pair.
The Three Core Numbers
Look along the inside of either temple arm (the piece that hooks over your ear). You’ll see a sequence that looks something like 52-18-140. Those three numbers tell you everything about the frame’s dimensions.
- Lens width (first number): The horizontal width of each lens, not including the frame material around it. This typically falls between 40 and 60 mm. A smaller number means a narrower lens, which generally suits a narrower face.
- Bridge width (second number): The shortest distance between the two lenses, right where the frame sits on your nose. This ranges from about 14 to 24 mm. If your glasses slide down or pinch, the bridge width is usually the culprit.
- Temple length (third number): The full length of the arm, measured from the hinge screw to the curved tip behind your ear. Most frames fall between 120 and 150 mm. Too short and the arms won’t reach comfortably behind your ears; too long and the frames will feel loose.
So a frame marked 52-18-140 has 52 mm lenses, an 18 mm bridge, and 140 mm temple arms. Every manufacturer uses this same sequence.
The Small Square Symbol
You might notice a small square (□) between the first two numbers instead of a dash, like 48□18. That square isn’t decorative. It signals that the manufacturer is using the standardized “boxing system” for measuring lens width. In this system, the lens width represents the widest horizontal point of an imaginary rectangle drawn around the lens shape.
If you see a dash or slash instead of the square, the manufacturer may be using their own measurement method. In practice, the difference is usually tiny, around 1 mm, but it’s worth knowing if you’re comparing frames across brands and the numbers seem slightly off despite looking like the same size.
Other Numbers and Codes on the Temple
The three size numbers rarely appear alone. Most frames pack several pieces of identifying information onto that same temple arm. Here’s how to tell them apart.
A model number is typically an alphanumeric code tied to the brand. Ray-Ban uses codes like RB3447, Oakley uses OO4141, and so on. This identifies the exact frame style, which is useful when you want to reorder the same pair or find replacement parts.
A color code appears near the model number and identifies the specific finish and lens tint. On a Ray-Ban, 001/51 means a gold frame with crystal brown lenses. These codes vary entirely by manufacturer, so there’s no universal system to decode them without checking the brand’s catalog.
Putting it all together, a Ray-Ban temple might read: RB3447 001/51 50-21-145 3N. That breaks down to model number, color code, lens width, bridge width, temple length, and a lens category code indicating tint darkness. The size numbers are always the trio of measurements in the familiar format.
Regulatory Markings
You may also spot a “CE” or “UKCA” marking on the inner temple. These aren’t size related at all. They indicate the manufacturer has declared that the frames meet health, safety, and performance requirements for sale in Europe or the United Kingdom. Most quality eyewear carries one or both of these marks, and you can safely ignore them when figuring out your frame size.
The Measurement That’s Not Printed
One important dimension is almost never stamped on the frame itself: the lens height, sometimes called the “B measurement.” This is the vertical height of the lens from top to bottom. It doesn’t appear in the standard three-number sequence, but it matters a great deal if you wear progressive or bifocal lenses. Those lenses need enough vertical space to fit the different viewing zones (distance at the top, reading at the bottom). A frame with a lens height under about 28 to 30 mm often won’t work well for progressives.
You’ll usually find the B measurement listed on a retailer’s product page or can ask an optician to measure it. If you’re buying frames online specifically for progressive lenses, check this number before ordering.
How to Use These Numbers When Shopping
The easiest way to find frames that fit is to start with a pair you already like. Read the numbers off the temple arm and use them as your baseline. When browsing new frames, look for lens widths within 2 mm of your current pair, bridge widths within 1 mm, and temple lengths within 5 mm. Small differences in lens and bridge width change how frames sit on your face more than you’d expect.
Lens width has the biggest impact on overall frame size. Jumping from 50 mm to 54 mm lenses doesn’t just make the lenses wider; it makes the entire frame noticeably larger on your face. Bridge width controls comfort and positioning. A bridge that’s too narrow will pinch and leave red marks, while one that’s too wide lets the frames slide forward. Temple length is the most forgiving of the three, since the curved ends can be adjusted by an optician to grip more or less tightly behind the ear.
If you don’t have a current pair to reference, you can measure your face directly. The distance across one eye, from the bridge of your nose to your outer eye socket, gives you a rough starting point for lens width. For bridge width, measure the flat area across the top of your nose where pads would rest. These won’t be exact, but they’ll narrow down your range enough to avoid obvious misfits when ordering online.