Sizing glasses comes down to three numbers printed on every frame: lens width, bridge width, and temple length, all measured in millimeters. These numbers tell you how wide the lenses are, how far apart the nose pads sit, and how long the arms are that hook behind your ears. You can find them printed on the inside of the temple arm (the part that goes over your ear), usually separated by dashes or squares, like 52-18-140.
What the Three Frame Numbers Mean
The first number is lens width, measuring one lens horizontally at its widest point. The second is bridge width, the gap between the two lenses that sits over your nose. The third is temple length, from the hinge to the tip that curves behind your ear. All three are in millimeters, and together they determine whether a frame fits your face or fights it.
Here’s how those numbers break down by general size category:
- Small: 47–49 mm lens width, 14–16 mm bridge, 135–140 mm temples
- Medium: 49–53 mm lens width, 17–19 mm bridge, 140–145 mm temples
- Large: 53–55 mm lens width, 19–21 mm bridge, 145–150 mm temples
If you already own a pair that fits well, check the inside of the arm for these numbers and use them as your starting point when shopping for new frames.
The Credit Card Trick for Quick Sizing
If you don’t have a well-fitting pair to reference, a standard credit card can give you a rough estimate. The short edge of a credit card is about 54 mm, which is close to the lens width of a medium frame. Hold the card vertically under one eye, with the long edge lined up with the center of your nose.
If the card extends past the outer corner of your eye, you likely need a smaller frame with narrower lenses. If it lines up right at the edge of your eye, medium frames are a safe bet. If the card doesn’t reach the outer corner, look at larger frames with wider lenses.
Choosing the Right Bridge Width
Bridge width matters more than most people realize. A bridge that’s too narrow pinches your nose and leaves red marks. One that’s too wide lets the glasses slide down constantly. Average nose bridge sizes range from 14 mm to 24 mm.
If you have a narrower or lower nose bridge, look for frames in the 16–18 mm range. These sit closer together and grip more securely, which also makes them a better choice if you’re active. For a wider or higher nose bridge, a 19–21 mm bridge distributes weight more evenly across a broader surface and won’t pinch. Many brands now offer “low-bridge fit” versions of popular frames, which have a wider bridge, longer nose pads, and adjusted curvature to prevent the frame from resting on your cheeks.
Measuring Your Pupillary Distance
Pupillary distance (PD) is the space between the centers of your two pupils, measured in millimeters. It’s not a frame measurement, but you need it whenever you order prescription lenses so the optical center of each lens lines up with your eyes. The average PD for women is around 60 mm, and for men about 64 mm, with a normal range of 48 to 73 mm.
With a Friend
Have them sit down and focus on something 10 to 20 feet away to keep their gaze steady. Place a millimeter ruler against their forehead, align the zero with the center of one pupil, and read the number at the center of the other pupil. That number is their PD.
By Yourself
Stand about 8 inches from a mirror. Hold a millimeter ruler horizontally across the bridge of your nose, pressing it lightly against your forehead for stability. Align the zero mark with the center of your left pupil, then close or cover your left eye. The number that lines up with the center of your right pupil is your PD. It helps to do this three times and average the results, since small head movements can shift the reading by a millimeter or two.
Frame Height for Progressive Lenses
If you wear progressive (no-line bifocal) lenses, frame height becomes a critical sizing factor. Progressives blend distance, intermediate, and reading zones into a single lens from top to bottom, and each zone needs enough vertical space to be usable.
The minimum recommended lens height for progressives is 28 mm, but 32 mm or more gives you the most comfortable experience with smooth transitions between zones. Frames that are too shallow cut into the reading portion at the bottom, forcing you to tilt your head at awkward angles to see things up close. If you’re choosing between two frames and you wear progressives, go with the taller lens every time.
Signs Your Glasses Don’t Fit
Even with the right numbers on paper, fit can vary between brands and styles. These are the clearest signals that something is off:
- Constant sliding: The frames slip down your nose repeatedly, meaning the bridge is too wide or the temples are too loose.
- Pain behind the ears: Pressure or soreness where the temple tips curve means the temples are too short or too tightly bent.
- Red marks on your nose: Deep indentations from the nose pads indicate the bridge is too narrow or the frame is too heavy for its contact points.
- Frames sitting too high or too low: Your glasses should rest in the middle of your face, no higher than your eyebrows. If the top of the frame covers your brows or the bottom sits on your cheeks, the lens height or bridge angle needs adjusting.
The overall width of the frame should match the width of your face at the temples. If you can see gaps between the sides of the frame and your head, the frame is too wide. If the frame digs into your temples or leaves marks, it’s too narrow.
Putting It All Together
Start with the numbers from a pair you already like, or use the credit card method to estimate your lens width. Measure your PD so you’re ready when ordering lenses. Pay attention to bridge width based on the shape of your nose, and if you need progressives, make sure the lens height hits at least 28 mm. Once you have those four measurements (lens width, bridge width, temple length, and PD) you can shop for glasses online or in-store with confidence that the frames will actually fit when they arrive.