The numbers on your glasses are size measurements printed in millimeters. You’ll find up to three numbers on the frame itself, and they tell you the lens width, bridge width, and temple length. If you also have a prescription card, that contains a separate set of numbers describing your lens power. Here’s what all of them mean.
The Three Numbers on Your Frame
Look on the inside of one temple arm (the piece that goes over your ear) or on the bridge (the piece that rests on your nose). You’ll see a sequence of numbers that typically looks something like 52-18-140. Each number is measured in millimeters.
- First number (lens width): The horizontal width of each lens opening, sometimes called “eye size.” This measures only the clear lens you look through, not the surrounding frame material.
- Second number (bridge width): The shortest distance between the two lenses, spanning the part of the frame that sits on your nose.
- Third number (temple length): The full length of each arm, from the hinge screw to the tip that curls behind your ear.
These three measurements are the universal sizing system for eyeglass frames. Every manufacturer uses them, so once you know your numbers, you can compare fit across brands.
What Counts as Small, Medium, or Large
Lens width is the primary indicator of overall frame size. Frames with a lens width of 50 mm or under are considered narrow and suit faces that measure less than 129 mm from temple to temple. A lens width of 51 to 55 mm fits a medium face (130 to 139 mm wide), and anything 56 mm or above is a wide frame for broader faces.
Bridge width follows a similar scale. A small bridge runs 14 to 16 mm, medium is 17 to 19 mm, and wide is 20 to 23 mm. Getting the bridge right matters more than most people realize. Too narrow and the frames pinch your nose; too wide and they slide down constantly.
Temple length is less variable. Most adult frames fall between 135 and 145 mm. If your current pair feels like it’s squeezing behind your ears or barely reaching them, check this number first when shopping for replacements.
The Numbers on Your Prescription
Your prescription is a separate set of numbers, usually printed on a card or slip from your eye doctor. It describes the corrective power your lenses need, measured in units called diopters. Each eye gets its own row, labeled OD (right eye) and OS (left eye), with several columns.
SPH (Sphere) is the main correction for your vision. A minus sign means you’re nearsighted and need help seeing things far away. A plus sign means you’re farsighted and need help with close-up vision. The higher the number in either direction, the stronger the correction. A prescription of -1.00 is mild; -6.00 is strong.
CYL (Cylinder) corrects astigmatism, a condition where part of your cornea has a slightly different curve than the rest. Not everyone has a CYL number. If yours is blank or zero, you don’t have astigmatism that needs correcting.
Axis always accompanies the CYL number. It’s written in degrees between 1 and 180 and tells the lab exactly where on your cornea the astigmatism sits, so the lens correction lines up in the right orientation.
ADD is an additional magnification power for reading, typically prescribed for people over 40 whose eyes have lost some ability to focus up close. This number is used in bifocals and progressive lenses. The lower portion of the lens carries the ADD power so you can read without switching to a separate pair of glasses.
Pupillary Distance
You may also see a number labeled PD on your prescription, though it’s not always included. Pupillary distance is the space between the centers of your pupils, measured in millimeters. The average adult PD is about 63 mm, but it ranges from roughly 50 to 70 mm for most people.
This number tells the lab where to position the optical center of each lens so it lines up with your eyes. If PD is off, even a perfect prescription can cause eye strain or blurry vision. Some doctors don’t write it on the prescription because federal rules don’t require it, but you can ask for it or have it measured at an optical shop.
Lens Height and Why It Matters
One measurement you won’t always see printed on the frame is the B measurement, which is the vertical height of the lens opening. This number becomes important if you’re getting progressive or bifocal lenses, because those lenses need enough vertical space to fit multiple vision zones.
Most optical professionals recommend a minimum lens height of 28 mm for standard progressive lenses. Specialized “short corridor” progressives can work in frames as small as 22 mm, but you’ll get a more comfortable experience with more room. If you’re shopping for frames and plan to use progressives, check the B measurement before falling in love with a pair of stylish narrow frames that won’t accommodate your lenses.
How to Measure if the Numbers Have Worn Off
The printed numbers on glasses can fade over time, especially on the temple arms. If yours are unreadable, you can measure the frame yourself with a millimeter ruler.
For lens width, measure the widest horizontal distance across one lens opening. For bridge width, measure the shortest gap between the two lens openings at the nose. For temple length, measure from the hinge to the very tip of the arm. If you’re measuring with the lenses still in the frame, add about 1 mm to your lens width and height measurements to account for the portion of the lens that tucks into the rim.
These measurements are taken from the inner edges of the frame groove (where the lens sits), not from the outer edges of the frame itself. Using a small plastic ruler with millimeter markings gives you more precision than a tape measure.