Measuring for glasses involves two categories of numbers: your pupillary distance (the space between your pupils) and your frame dimensions (the physical size of the glasses). You need both to order glasses that fit your face and align correctly with your eyes. Most of these measurements can be taken at home with a millimeter ruler and a mirror.
Pupillary Distance: The Most Important Number
Pupillary distance, or PD, is the distance in millimeters between the centers of your two pupils. This measurement tells the lens maker exactly where to place the optical center of each lens so it lines up with your eye. If it’s off, you may experience eye strain, headaches, or blurry vision. The average adult PD falls between 54 and 74 mm, while children typically range from 43 to 58 mm.
There are two types of PD. A single PD is one number representing the total distance between both pupils, like 63 mm. A dual PD (also called monocular PD) gives a separate measurement for each eye, measured from the center of each pupil to the bridge of your nose, written as something like 31.5/30 mm. Dual PD is more precise because most faces aren’t perfectly symmetrical. It’s especially important for progressive or multifocal lenses, where even a small misalignment affects how well the different lens zones work.
How to Measure PD at Home
Stand about 8 inches from a mirror in a well-lit room. Hold a millimeter ruler flat against your brow, resting it on the bridge of your nose. Close your right eye and align the ruler’s zero mark with the center of your left pupil. Then, keeping the ruler still, close your left eye and open your right. Read the millimeter mark that lines up with the center of your right pupil. That number is your single PD.
For dual PD, you measure each eye separately. The process is the same, but instead of reading from one pupil to the other, you read from each pupil to the center of your nose bridge. You’ll end up with two numbers, one for each eye.
Repeat the measurement two or three times. If your results vary by more than a millimeter, keep going until you get consistent readings. Having someone else hold the ruler while you look straight ahead at a distant point often produces more reliable results than doing it solo in a mirror.
If you’re ordering reading glasses, subtract 3 mm from your distance PD to get your near PD. So a distance PD of 63 mm becomes a near PD of 60 mm. For dual PD, subtract 1.5 mm from each eye’s number.
Smartphone Apps as an Alternative
Several smartphone apps measure PD using your phone’s camera. A study comparing popular PD apps against professional equipment found that the best-performing app produced results within 0.2 mm of clinical measurements, which is accurate enough for most prescriptions. These apps typically ask you to hold a credit card or standard reference object near your face so the software can calibrate for scale. They’re a reasonable alternative if you find the ruler method difficult, though repeating the measurement a few times is still smart.
Understanding Frame Numbers
Every pair of glasses has a sequence of numbers printed on the inside of the temple arm (the piece that goes over your ear). These three numbers, always in millimeters, represent the same three measurements in the same order across virtually every brand: lens width, bridge width, and temple length. A frame stamped 52-18-140 has 52 mm wide lenses, an 18 mm bridge, and 140 mm temple arms.
If you already own a pair of glasses that fits well, these numbers are the fastest shortcut to finding your size. Just read them off the frame and use them as your baseline when shopping.
How to Measure Frame Width on Your Face
If you don’t have a well-fitting pair to reference, you can measure your face directly. Hold a ruler horizontally across your face just below your eyes and measure the distance between your left and right temples. This gives you your total frame width. When comparing to frames online, the total frame width is roughly equal to twice the lens width plus the bridge width, so you can check whether a specific pair will be too narrow or too wide.
Bridge Width and Nose Fit
The bridge is the small arch that sits on your nose, and getting this measurement right determines whether your glasses slide down, pinch, or sit comfortably. Bridge widths generally fall into three categories: small (14 to 16 mm), medium (17 to 19 mm), and large (20 to 23 mm).
A quick way to gauge your bridge size is to take a side-profile photo. If your nose bridge sits level with or below your pupils and is relatively narrow, you’ll likely need a smaller bridge number. If your bridge is above your pupils or wider than average, look for a larger number. People with a low or flat nose bridge often do better with frames specifically designed for a low-bridge fit, which have curved nose pads and a lower bridge connection point to prevent the frames from resting on your cheeks.
Temple Length
Temple length is measured from the hinge screw (where the arm connects to the front of the frame) all the way to the tip that hooks behind your ear. Most adult temples range from 135 to 150 mm. If your current glasses feel like they’re squeezing behind your ears or barely reaching, measure the temple arm of those frames and adjust up or down accordingly. Temples that are too short will press uncomfortably; too long and the frames will feel loose and slide forward.
Lens Height for Progressive Lenses
If you wear progressive, bifocal, or multifocal lenses, there’s one additional measurement that matters: segment height. This is the distance from the center of your pupil down to the bottom edge of the lens (not the frame). It tells the lab where to position the transition between your distance, intermediate, and reading zones.
To measure it, put on the frames you plan to use, look straight ahead into a mirror, and mark the center of each pupil on the lens with a small dot or tiny piece of tape. Then measure from each dot straight down to the very bottom of the lens. Record both eyes separately and repeat two or three times to confirm consistency.
This measurement is frame-specific. If you switch to a different frame, the segment height will almost certainly change because every frame sits differently on your nose and face. Getting it wrong has real consequences: if the segment is set too high, you’ll be looking through the reading portion more than you should, and if it’s too low, you’ll find yourself tilting your head awkwardly to read. For this reason, some people prefer to have segment height measured in person at an optical shop, even if they order their lenses online.
Measuring Children
Children’s measurements follow the same principles but require a bit more patience. Kids have a PD range of 43 to 58 mm, and the margin for error on a smaller face is tighter. The most reliable approach is to have your child sit still and look at a fixed point straight ahead while you place a millimeter ruler across the bridge of their nose. Read the distance between the center of each pupil. The key is making sure they’re looking directly forward, not at you, the ruler, or something off to the side, because even a slight eye turn will throw the number off.
Measure at least two or three times. With younger children, you may need to try on different occasions when they’re more cooperative. For frame sizing, children’s frames use the same three-number system as adult frames, just in smaller ranges. Prioritize bridge fit and temple length, since a child’s frame that constantly slides down will get bent, sat on, or ignored.