Every glasses frame has three numbers printed on it, usually on the inside of the temple arm, and those three numbers tell you almost everything you need to know about fit. They look something like 48-19-140 and represent lens width, bridge width, and temple length, all in millimeters. If you already own a pair that fits well, finding those numbers is the fastest path to your next pair. If you don’t, you can measure your face in a few minutes.
The Three Numbers on Every Frame
Look on the inside of your current glasses, usually along the temple arm or sometimes on the bridge. You’ll find three numbers separated by dashes or squares. Here’s what each one means:
- Lens width (first number): The horizontal width of each lens at its widest point, sometimes called “eye size.” Adult frames typically range from 40 mm to 60 mm.
- 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 that hooks over your ear. Most adult temples run 120 mm to 150 mm.
So a frame marked 52-18-140 has 52 mm lenses, an 18 mm bridge, and 140 mm arms. When shopping for new frames, matching these numbers within a millimeter or two of a pair you already like is the simplest way to get a reliable fit.
How to Measure Your Face
If you don’t have an existing pair to reference, start with overall frame width. Hold a ruler horizontally across your face, just below your eyes, and measure the distance from temple to temple. This gives you the total frame width you need. For most adults, that falls somewhere between 125 mm and 150 mm. A frame that matches your face width will sit flush at the temples without pressing into the sides of your head or extending past your face.
For lens width specifically, you can measure the horizontal distance across one eye, from the outside corner to the bridge of your nose. This approximates the lens size that will center your eyes in the frames. Most adults land between 50 mm and 58 mm, though people with narrower faces may need something in the mid-40s.
Getting the Bridge Right
Bridge width is the measurement people most often overlook, and it’s the one most responsible for glasses that slide down your nose or pinch. If you have a narrower nose or your eyes sit closer together, look for a bridge width between 14 mm and 18 mm. A wider nose or eyes set farther apart calls for 18 mm and above. When the bridge is too wide, the frames sit too low and slip. When it’s too narrow, they perch high on your nose and squeeze.
Checking Temple Length
Temple arms need to reach comfortably over your ears with a gentle curve. If they’re too short, the frames will feel tight and pull forward. If they’re too long, they’ll stick out past your ears and the glasses will feel loose. Most adults do well with 140 mm temples, which is the most common length. If you have a notably smaller or larger head, you might need 130 mm or 150 mm respectively. You can check by measuring from the hinge area of your current glasses along the arm to where it curves behind your ear, then adding the curve length.
Low Bridge Fit and Face Shape
Standard frames are designed for people whose nose bridge sits roughly in line with their pupils and who have lower cheekbones. If your nose bridge sits below your pupils, or if you have higher cheekbones, standard frames tend to slide down, rest on your cheeks, or sit too close to your face. In that case, look for frames labeled “low bridge fit” or “global fit.” These have a longer nose pad area, a slightly curved frame front, and adjusted temple angles that keep the glasses in place without relying on a prominent nose bridge for support.
Why Pupillary Distance Matters
Frame size gets the glasses on your face comfortably, but there’s a separate measurement that determines whether you actually see clearly through them: pupillary distance, or PD. This is the distance between the centers of your pupils, and it tells the lab where to place the optical center of each lens. When PD is even slightly off, the lenses don’t align with your natural line of sight, which can cause eye strain, headaches, and blurry vision.
PD becomes especially important with bifocals or progressive lenses, where the lens power changes across different zones. Misalignment in those lenses means the reading zone or distance zone won’t line up where your eyes naturally look. Your eye care provider measures PD during an exam, but many online retailers also walk you through measuring it yourself using a mirror and a millimeter ruler. The average adult PD is around 63 mm, but individual measurements range widely.
Size Ranges for Children
Kids’ faces change fast, so sizing by age group gives a useful starting point:
- Ages 0 to 2: Lens width 39 to 42 mm, bridge 14 to 16 mm, temples 115 to 125 mm
- Ages 2 to 5: Lens width 42 to 45 mm, bridge 15 to 16 mm, temples 125 to 130 mm
- Ages 5 to 8: Lens width 45 to 48 mm, bridge 16 to 17 mm, temples 130 to 135 mm
- Ages 8 to 12: Lens width 48 to 50 mm, bridge 16 to 18 mm, temples 135 to 140 mm
- Teens: Lens width 49 to 52 mm, bridge 16 to 18 mm, temples 140 to 150 mm
These are guidelines, not rules. A small-framed 10-year-old might still wear the 5 to 8 range, and a larger teen might already fit adult sizes. Measuring an existing pair that fits is always more reliable than going by age alone.
Signs Your Frames Don’t Fit
Even with the right numbers, it helps to know what poor fit looks and feels like. Frames that are too wide will slide when you look down and leave gaps at the temples. Frames that are too narrow press into the sides of your head and leave red marks. If the lenses extend past the edges of your face or your eyes sit off-center in the lenses, the frame width or lens size is wrong.
On the nose, you’ll feel a too-tight bridge as pinching at the top of your nose, while a too-loose bridge lets the glasses slide down within minutes of putting them on. Temple arms that are the wrong length cause the most frustration: too short and the glasses feel like they’re squeezing your head; too long and they never stay in place no matter how many times you push them up.
The ideal fit keeps the frames level on your face, centered on your nose, with your pupils sitting near the middle of each lens both horizontally and vertically. The arms should follow the contour of your head without pressure and curve gently over your ears without digging in.