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How do you measure glasses frame size?

By Spektrum Glasses Editorial Team · Published 2026-08-16 · Updated 2026-09-11 · Facts re-checked 2026-09-11 How this page is written and checked: our editorial method · how we verify claims

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Short answer

Frame size is stamped inside the temple arm as three numbers, such as 52-18-140: lens width, bridge width, and temple length in millimeters. The most reliable way to pick a size is comparing those numbers to a pair that already fits well, since S/M/L labels are not standardized between brands and can vary by 10 millimeters or more.

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Where the size numbers are printed

Look at the inside of either temple arm (the piece that goes over your ear) or along the bridge of the frame between the lenses. Most glasses have a small stamp or laser etching with a sequence like 52-18-140, sometimes separated by squares or dashes instead of hyphens. That sequence is the frame’s size code, and it is the single most useful piece of information for judging fit before you buy, more useful than a written size like “medium” or “universal fit.” If you cannot find a size code on a pair you already own, check the original packaging or the product listing description - it is usually printed there even when it has worn off the frame itself.

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What the three numbers mean

Each number in the sequence measures a specific dimension of the frame, always in millimeters: Add the two lens widths and the bridge width together and you get the frame’s total width, which is the number that correlates most closely with how the frame sits on your face. A 52-18-140 frame has a total width of about 122mm (52 + 52 + 18). Temple length mostly affects how far the arm reaches over the ear and matters more for comfort over a full day than for whether the frame looks proportional.

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Measuring your face instead of guessing

The size code only helps if you know what number you are aiming for. Two practical ways to get there:

• Measure a pair you already own and like the fit of. Use a millimeter ruler against the size code, or if it is worn off, measure lens width and bridge width directly with a ruler held flat against the front of the frame.
• Measure your own face width temple to temple (across the widest part, roughly eyebrow height) with a ruler or tape measure, then look for a frame whose total width is close to that number, generally within a few millimeters on either side.

A frame that is significantly narrower than your face width will pinch at the temples; one that is significantly wider will slide down your nose and may extend past the outer edge of your face. Neither is a matter of preference so much as basic fit, and it is worth checking before ordering rather than after.

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What the size code does not tell you

The three numbers describe the frame’s outer dimensions, not how it will feel. They do not capture nose bridge shape (a low or high bridge fits differently even at the same bridge-width number), lens curvature, frame weight, or how the temple tips curve behind the ear. Two frames with an identical 52-18-140 code can fit noticeably differently depending on these factors, which is why matching a pair you have already worn is more reliable than matching numbers alone. Pupillary distance (PD), the measurement between the centers of your two pupils, is a separate figure used to center prescription lenses in a frame. It has no bearing on non-prescription glasses, including non-prescription reading and blue-light styles, since there is no optical center that needs to align with your eyes for a single-vision, non-prescription lens. For non-prescription reading glasses specifically, the frame mold and its size code stay the same across different magnification strengths (for example 0 through +3.00). The lens power changes the curve ground into the lens, not the frame’s outer dimensions, so if a given frame size fits well at one strength it will fit the same way at any other strength in that same style.

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A quick checklist before buying

• Find the size code (lens-bridge-temple, in millimeters) on a pair that currently fits you well, or in the product listing.
• Compare total width (lens width x2 + bridge width) against your face width, not just the lens width alone.
• Note whether the nose pads are adjustable silicone or fixed plastic; a fixed bridge depends more heavily on getting the bridge-width number right.
• Check temple length against your ear-to-ear-plus-a-bit distance if you wear glasses for many hours at a time, since a too-short temple is a common source of pinching.
• Remember PD only matters if you are ordering a prescription lens; skip it for non-prescription frames.

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Where these numbers come from

Every measured figure quoted here is transcribed from a third-party laboratory report, published in full with the wavelength band and the report number: lab results. Our rule for what may appear on this page at all is on how we choose what to publish.

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Related questions

• How do you choose blue light glasses?
• How much should blue light glasses cost?
• Can you wear blue light glasses all day?
• Can you wear blue light glasses over contact lenses?
• Can you wear blue light glasses over prescription glasses?
• Are clip-on blue light filters any good?

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