Every pair of glasses has three numbers printed on it that tell you exactly what size it is. Finding those numbers on a pair that already fits you well is the fastest way to know your size. If you don’t have a pair to reference, you can figure out your measurements using a ruler and a few simple guidelines.
The Three Numbers That Define Frame Size
Look at the inside of either temple arm (the piece that extends back to your ear) on any pair of glasses. You’ll find a series of numbers that typically looks something like 48-19-140. Those three numbers, all in millimeters, are your frame size.
- Lens width (first number): The horizontal width of each lens at its widest point. This ranges from about 40 mm to 60 mm and is the single biggest factor in how a frame looks on your face.
- Bridge width (second number): The gap between the two lenses, which sits on your nose. This ranges from 14 mm to 24 mm.
- Temple length (third number): The full length of each arm, from the hinge to the curved tip behind your ear. The most common options are 135, 140, 145, and 150 mm.
The numbers are sometimes separated by a small square symbol instead of dashes. If you already own glasses that fit comfortably, write down all three numbers. They’re your baseline for shopping, especially online.
What Each Measurement Means for Fit
Lens width controls how wide the frame sits across your face. As a general guide, frames with a lens width of 47 to 49 mm suit smaller faces, 49 to 53 mm fits medium faces, and 53 to 55 mm works for larger faces. If the frame extends noticeably past the sides of your face, the lens width is too large. If your face is wider than the frame, it’s too small.
Bridge width determines whether the glasses slide down your nose or pinch it. A bridge that’s too narrow will leave red marks on the sides of your nose. One that’s too wide will cause the frames to slip forward constantly, which also throws off your lens alignment. Even a 2 mm difference in bridge width can change the fit dramatically, so pay close attention to this number if you’ve ever struggled with glasses sliding.
Temple length affects comfort behind your ears. Arms that are too short won’t reach far enough and will feel like they’re squeezing your head. Arms that are too long will stick out past your ears and won’t hold the frames securely. Most adults fall in the 140 to 145 mm range.
How to Measure Without an Existing Pair
If you don’t have glasses to reference, you can estimate your size with a standard ruler or a millimeter tape measure. Stand in front of a mirror, hold the ruler across your face at eye level, and measure the distance from one temple (the flat area beside your eye) to the other. This total face width, divided roughly in half and then reduced slightly to account for the bridge and frame material, gives you a ballpark lens width.
For bridge width, measure the widest part of your nose where glasses would rest. Most people don’t need to be precise here to the millimeter. Just knowing whether you’re at the narrow end (14 to 17 mm) or wider end (18 to 24 mm) of the range helps you filter options. If you have a flatter or wider nose bridge, or if the top of your nose bridge sits at or below your pupils, you may want to look specifically for “low bridge fit” frames. These sit lower on the face, have larger nose pads, and angle outward more at the temples to prevent the frame from resting on your cheeks.
Why Pupillary Distance Matters
Beyond the three frame numbers, there’s a fourth measurement that affects your vision more than any of them: pupillary distance, or PD. This is the distance in millimeters between the centers of your pupils. It tells the lab where to place the optical center of each lens so it lines up with your eyes.
When PD is off by even a small amount, the lens centers don’t match your natural line of sight. This can introduce subtle distortion, eye strain, headaches, and blurry vision, especially with stronger prescriptions. Your eye care provider can measure this during an exam, and some online retailers offer tools to measure it at home using a webcam or a ruler and mirror. The average adult PD falls between about 54 and 74 mm, though yours may sit outside that range.
Choosing Frames for Progressive Lenses
If you wear progressive (multifocal) lenses, frame size gets an extra requirement: the lens needs to be tall enough vertically to fit all three vision zones (distance at the top, computer-range in the middle, reading at the bottom). The vertical height of the lens, sometimes called the B-measurement, should be at least 28 mm for standard progressives. Below that, the reading zone at the bottom gets cut off or compressed, which makes close-up tasks frustrating.
Short-corridor progressive designs can work in frames as small as 22 mm vertically, but most optical professionals recommend 28 to 32 mm for comfortable, full-range vision. If you spend a lot of time on a computer, office or computer-specific progressive lenses work best in frames with a vertical height of 30 mm or more. Keep this in mind before choosing a narrow, rectangular frame style if you need multifocal lenses.
Signs Your Current Glasses Don’t Fit
Sometimes the easiest way to figure out the right size is to identify what’s wrong with your current pair. Glasses that are too tight leave marks on your nose or behind your ears and cause headaches by the end of the day. Glasses that are too loose slide down when you look down, require constant pushing back up, and may sit crooked on your face.
Less obvious signs include tired eyes, neck pain, or mild nausea. These can happen when the lenses aren’t sitting at the right distance or angle in front of your eyes, forcing your visual system to work harder to focus. Progressive lens wearers are especially prone to this: poorly fitting progressives can cause dizziness on stairs or when shifting your gaze between zones. If you’re experiencing any of these symptoms and your prescription is current, the problem is likely fit rather than prescription strength.
The lenses should sit close to your face without touching your eyelashes or cheeks. Your eyes should appear roughly centered in each lens, not pushed to one side or sitting near the top or bottom edge. And the frame should follow the line of your brow without sitting significantly above or below it.
Practical Tips for Online Shopping
Buying glasses online makes size even more important since you can’t try them on first. Start by recording the three numbers from a pair that fits you well. When browsing, filter by lens width first, since it has the biggest impact on overall fit. Stay within 2 mm of your known lens width and 1 mm of your bridge width for the safest results.
Many online retailers now offer virtual try-on features that overlay frame images on a photo or live video of your face. These are useful for judging style but unreliable for precise fit, so always cross-check the millimeter measurements. If you’re between sizes, going slightly larger in lens width is usually more comfortable than going smaller, as long as the frames don’t extend past the sides of your face. And confirm the return policy before ordering, since even careful measuring occasionally results in a pair that doesn’t feel right once it’s on your face.