The numbers on your glasses are size measurements in millimeters. You’ll typically find three numbers printed on the inside of the temple arm (the part that goes over your ear), and they always appear in the same order: lens width, bridge width, and temple length. A marking like 50-21-145 means each lens is 50 mm wide, the bridge spanning your nose is 21 mm, and each arm is 145 mm long.
Lens Width: The First Number
The first number, sometimes called “eye size,” is the horizontal width of each lens. It typically ranges from 40 mm to 60 mm. This is the single most important number for determining how a frame fits your face. A smaller lens width suits narrower faces, while wider faces generally need something in the 52-58 mm range.
Lens width also affects how your prescription lenses are made. If you’re getting progressive or bifocal lenses, the lens needs enough vertical height to fit the different vision zones. That vertical measurement, sometimes listed as a fourth number, isn’t always printed on the frame but matters when ordering lenses online.
Bridge Width: The Second Number
The second number measures the shortest distance between the two lenses, right where the frame sits on your nose. Bridge width ranges from 14 mm to 24 mm. Getting this number right is the difference between glasses that stay put and glasses you’re constantly pushing back up.
A bridge that’s too wide will slide down your nose. One that’s too narrow will pinch or perch awkwardly high. When weight is concentrated on a small area of the bridge, it creates pressure marks on the skin and ongoing discomfort. If you have a higher nose bridge, a snug but not squeezing fit keeps the frame stable. If your nose bridge is flatter or lower, you’ll generally need a smaller bridge measurement or frames designed with nose pads that can be adjusted.
Temple Length: The Third Number
The third number is the length of each arm, measured from the hinge screw all the way to the tip that curls behind your ear. Standard temple lengths run from 120 mm to 150 mm, with 140 mm being the most common for adults.
The key to a good temple fit isn’t just overall length. It’s where the arm bends. The bend should sit right at the top of your ear, resting snugly without pressing. If the temples are too short, they’ll squeeze behind your ears and cause soreness. If they’re too long, they won’t grip properly and the frame will slide forward, putting extra pressure on your nose. Ideally, the arms shouldn’t touch the sides of your head at all before reaching your ears. If they do, the frame is too tight through the temples and will constantly slip forward.
Other Numbers and Codes on the Frame
You’ll often see more than just three numbers on your temple arms. Manufacturers print model numbers, color codes, and brand identifiers in the same area, which can make things confusing. Here’s how to tell them apart.
Take a Ray-Ban frame marked “RB3447 001/51 50-21 145 3N.” The three size numbers are 50, 21, and 145, usually separated by dashes or a small square symbol. Everything else is manufacturer information: RB3447 is the model name, 001/51 is the color code (gold frame with brown lenses, in this case), and 3N refers to the lens tint category. Oakley uses a similar pattern: “OO4141-01 60◻14 132” means model OO4141 in color variant 01, with a 60 mm lens, 14 mm bridge, and 132 mm temple.
The easiest way to spot the size numbers is to look for the cluster of two- and three-digit numbers. The two smaller numbers (typically in the 40-60 and 14-24 range) are lens and bridge width. The three-digit number in the 120-150 range is temple length.
Safety and Compliance Markings
Some frames carry letters or symbols that have nothing to do with size. The most common is “Z87” or “Z87+,” which indicates safety-rated eyewear meeting the ANSI impact protection standard used in the United States. Frames marked Z87-2 meet general-purpose protection requirements. If you see a “+” symbol on both the lenses and the frame, that means the eyewear is rated for high-impact protection. These markings matter if you’re buying glasses for workplace safety or sports.
Ready-made reading glasses are also marked with their magnification power (like +1.50 or +2.00), which tells you the lens strength rather than the frame size.
How to Use These Numbers When Buying Glasses
If you already own a pair of glasses that fits well, the numbers on those frames are your starting point. Look on the inside of the temple arm or, less commonly, on the inside of the bridge. Write down all three numbers before shopping for a new pair.
When comparing frames, you don’t need an exact match on every measurement. A difference of 1-2 mm in lens width or bridge width is barely noticeable. Temple length matters more in discrete jumps, since most manufacturers only offer a few options (135, 140, 145, or 150 mm). Prioritize getting the bridge width close to your current pair, since that has the biggest impact on comfort and how well the glasses stay on your face.
If you’re buying online without a reference pair, you can measure your face directly. Use a ruler or measuring tape across one eye to estimate lens width, measure the flat part of your nose bridge for bridge width, and measure from your temple to the top of your ear for a rough temple length. These won’t be precise, but they’ll get you into the right range.