Glasses are measured using a few key numbers, most of which are printed right on the frame itself. The three standard measurements are lens width, bridge width, and temple length, typically stamped in millimeters on the inside of one temple arm. If you’re ordering glasses online or replacing a pair that fits well, these numbers are your starting point.
The Three Numbers on Your Frames
Look at the inside of your glasses’ temple arm (the piece that hooks over your ear) or along the bridge. You’ll find a set of numbers that looks something like 48□18-140. Each number describes a different part of the frame.
- Lens width (eye size): The first number is the widest horizontal measurement across one lens, in millimeters. In the example above, that’s 48 mm.
- Bridge width: The second number is the distance between the two lenses at their closest point, not the full width of the nose piece. In this case, 18 mm. This determines how the glasses sit on your nose.
- Temple length: The third number is the full length of the arm from hinge to tip, usually between 135 and 150 mm. This affects how the frame fits around your ears.
The small square symbol (□) between the first two numbers is part of a standardized system called the boxing system. You don’t need to worry about its meaning. Just know that the first number is lens width and the second is bridge width.
How to Measure Frames With a Ruler
If the printed numbers have worn off, or you want to double-check them, you can measure an existing pair at home with a millimeter ruler. Place the glasses face-up on a flat surface with the arms fully extended. For bridge width, measure the gap between the two lenses at the narrowest point, keeping the ruler parallel to the top edge of the lenses and following the natural curve of the bridge.
For lens width, measure the widest horizontal distance across one lens, from the inner edge of the rim to the outer edge. This should match the first number printed on the frame. If you’re comparing frames from different brands, keep in mind that a difference of just 1 to 2 mm in bridge width can noticeably change how the glasses feel on your nose.
Lens Height: Why It Matters for Progressive Lenses
There’s a fourth measurement that doesn’t always appear on the frame but becomes critical if you wear progressive or bifocal lenses. The “B measurement” is the vertical distance from the very top to the very bottom of the lens opening. Progressive lenses need enough vertical space to fit all three vision zones (distance, intermediate, and reading), so most opticians recommend a B measurement of at least 28 to 30 mm for standard progressives.
If you’re ordering progressives online, measure this yourself by holding a ruler vertically against one lens from top rim to bottom rim. Frames with shallow, narrow lenses may not provide enough room for comfortable progressive vision.
Measuring Your Face for New Frames
If you don’t have a pair that fits well already, you’ll need to measure your face directly. The most important dimension is face width, which determines how wide your frames should be overall. Hold a ruler horizontally across your face just below your eyes and measure the distance between your left and right temples. This number, typically between 120 and 150 mm, tells you the total frame width to look for.
Frames that are too narrow will pinch and leave red marks. Frames that are too wide will slide down your nose and sit unevenly. As a general rule, the edges of the frame should line up roughly with the widest part of your face.
Measuring Your Pupillary Distance
Pupillary distance (PD) is the distance between the centers of your two pupils, and it determines where the optical center of each lens is placed. The average adult PD is about 63 mm, but it ranges from roughly 50 mm to 70 mm. Getting this number wrong by even a few millimeters can cause eye strain, blurry vision, or headaches, especially with stronger prescriptions.
Your eye doctor may include PD on your prescription, but many don’t. You can measure it yourself at home with a millimeter ruler and a mirror:
- Stand about 8 inches from a mirror and look straight ahead.
- Hold the ruler flat against your brow line.
- Close your right eye and align the ruler’s 0 mm mark with the center of your left pupil.
- Open your right eye and close your left eye.
- Read the millimeter mark that lines up with the center of your right pupil. That number is your PD.
The reason you alternate closing each eye is to avoid parallax error. If both eyes are open while you’re reading the ruler, your gaze shifts slightly and throws off the measurement. Repeat the process two or three times and take the average.
Smartphone Apps vs. Manual Measurement
Several apps now use your phone’s camera to calculate PD. A study of 178 participants published in Investigative Ophthalmology & Visual Science compared two popular apps against a manual ruler and a professional pupillometer. The apps were actually more repeatable than manual measurement: their readings varied by less than 0.3 mm between sessions, compared to about 0.75 mm for a hand-held ruler. However, the apps tended to read slightly lower (by roughly 0.7 mm on average) than manual tools.
In practical terms, both methods are accurate enough for most prescriptions. If your prescription is strong (above +/- 4.00 diopters) or you’re ordering progressive lenses, it’s worth measuring your PD multiple times or having an optician verify it. A millimeter or two of error matters more as lens power increases.
Putting It All Together
When you’re shopping for glasses online, here’s what you need to have on hand: lens width, bridge width, and temple length from a well-fitting pair (or your face measurements if you’re starting from scratch), your PD, and, if you wear progressives, the lens height. Most online retailers let you filter by these dimensions, so matching a new frame to your measurements is straightforward once you have the numbers. If you’re between sizes, prioritize getting the bridge width right. That single measurement has the biggest impact on comfort and how the frame sits on your face.