Where are the size numbers on my glasses?

Look at the inside of the temple arms, usually the left one. If they are not there, check behind the bridge or on a nose pad arm. The stamp reads like 54-17-140, sometimes with a small square between the first two numbers.

What does the square symbol in the size stamp mean?

The square means the lens was measured with the boxing system: the width of a rectangle drawn tight around the lens. It separates lens width from bridge width in the stamp. It does not change the meaning of the numbers.

Blog · Updated 15 Sep 2026 · Fit

Glasses frame numbers explained: 54-17-140

Q: Can two frames with the same numbers fit differently?

Yes. The numbers describe width, not shape. Lens height, wrap, weight, and nose pad design all change how a frame sits. Use the numbers to shortlist, then look at the frame on your own face before you buy.

Q: What frame width should I choose for my face?

Measure the total width of glasses that already fit you well and compare it with the manufacturer's total-width measurement. When total width is missing, two lens widths plus the bridge plus an approximate allowance for the end pieces can provide a rough estimate.

A stamp such as 54□17 140 gives three measurements in millimetres: lens width, bridge width and temple length. It helps you shortlist a size—but it does not guarantee the frame will fit.

Inside the left temple arm

54Lens widthOne lens at its widest point

17Bridge widthThe gap across your nose

140Temple lengthThe arm from hinge to tip

Every number is in millimetres. The square is a separator, not another measurement.

The three numbers

Number	Name	Typical range
54	Lens width	40 - 62 mm
17	Bridge width	14 - 24 mm
140	Temple length	120 - 150 mm

Lens width is the horizontal width of one lens at its widest point. Changing it usually has the largest effect on the front of the frame.

Bridge width is the distance between the lenses. Too narrow can pinch; too wide can let the frame sit low or slide. Shape and nose-pad design matter as well as the printed number.

Temple length is the arm from hinge to tip. Most adult frames fall between 135 and 145 mm. The bend should begin just past your ear rather than pressing before it or extending far behind it.

The small square between the first two numbers refers to the boxing measurement system. Some frames also print lens height. That fourth measurement is especially useful when checking the minimum lens height for progressive lenses.

The number that is not printed: total width

The stamp usually does not state the frame's total width. Product listings sometimes do, and that manufacturer measurement is the best one to use. When it is missing, this rough estimate is useful:

54left lens+54right lens+17bridge+~12end pieces=~137 mmestimated total width

The extra 12 mm is only an allowance for the two end pieces around the hinges; it varies by frame design. Compare the estimate with a well-fitting frame you already own, or use the manufacturer's stated total width. Matching only the three-number stamp is not enough.

How to use the numbers when you buy online

Start with a frame that already fits.Photograph its stamp so you have a reliable baseline.
Match lens and bridge first.Stay close to both measurements instead of matching just one number.
Check total width.Prefer the manufacturer's measurement; otherwise use the estimate above.
Check lens height and shape.They affect your cheeks, eyebrows, prescription and field of view.
Use a try-on as the final check.The numbers shortlist. Your face decides.

Where the numbers stop helping

Two frames with identical stamps can sit very differently. The numbers do not capture lens shape, frame wrap, end-piece width, weight, nose-pad position or where the temples flare. They also say nothing about whether the frame clears your cheeks and follows your brow.

Use the stamp to narrow the search—not to declare a fit.A good result should sit level, keep your pupils comfortably inside the lenses, clear your cheeks and avoid pressure at the nose and temples.

Check the whole fit Turn a frame listing into a fit verdict

Paste a glasses link into SpecFit. It reads the listed size, compares it with your measured face and explains the result in plain language.

Download SpecFit

Questions

Where are the numbers on my glasses?

Inside a temple arm, usually the left. If not there, check behind the bridge or on a nose pad arm. Worn stamps rub off; a photo of the frame in good light often reveals them.

What does the square symbol mean?

It separates lens width from bridge width and signals that the lens was measured using the boxing system. It is not a fourth measurement.

Can two frames with the same numbers fit differently?

Yes. Lens shape, total width, wrap, end pieces and nose-pad design can differ even when the stamp matches. Use the numbers to shortlist and your face to decide.

What frame width should I choose for my face?

Start by measuring the total width of glasses that already fit you well and compare that with the product listing. That is more reliable than trying to infer fit from lens width alone.

What if I do not know my face width?

Measure temple to temple with a ruler, or let a depth scan measure it along with your PD. Comparing against a frame that already fits is also a useful starting point.

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