Sunglasses size guide

Sunglasses are sized by three numbers printed inside the temple arm: lens width, bridge width and temple length, all in millimetres. A frame stamped 52 □ 18 140 has a 52 mm lens, an 18 mm bridge and 140 mm arms. Those three figures are the whole sizing system.

What do the three numbers on sunglasses mean?

The three numbers appear in a fixed order and are always millimetres. The order does not change between brands.

The small square between the first and second number is a printers' mark for the boxing system, the measuring convention that defines where each dimension is taken from. It is not a separate measurement. Some frames use a dash or a space instead.

The ranges above describe where most adult frames sold today land. They are not a standard, and oversized, narrow and children's styles sit outside them on purpose. Treat them as orientation, not as a rule to shop by.

Where are the numbers printed on a frame?

Look on the inside face of the left temple arm, near the hinge. That is the usual place. Some brands print them on the inside of the bridge instead, and some put them on the nose pad arm of a metal frame. On dark acetate the figures are often engraved rather than inked, so tilt the frame under a light to catch them.

Alongside the three sizing numbers you will often see a model number and a colour code. Those are catalogue references, not measurements. If you only find two numbers, you have almost certainly found the model code, not the size.

If nothing is printed at all, measure the frame yourself. How to measure sunglasses covers where to put the ruler for each of the three dimensions. For a closer read of what each figure controls, see what the numbers on sunglasses mean.

What is a normal sunglasses size?

There is no single normal. Lens width is largely a styling decision: a 62 mm lens is not a bigger version of a 52 mm lens, it is a different look. The two numbers that decide whether a frame stays put are bridge width and temple length.

Bridge width governs where the frame rests. Too narrow and the frame sits high and pinches; too wide and it slides down the nose. Temple length governs how far the arm reaches before it turns down behind the ear. A temple that is too short pulls the frame forward; too long and the arm has nothing to hook onto.

How do I use a frame I already own?

The most reliable sizing reference you have is a pair that already sits well. Read its three numbers and use them as the target. This is the same logic ShirtMath applies to shirts: compare a known good garment against published measurements rather than guessing from a letter.

1. Find your reference pair — the sunglasses or spectacles you wear without adjusting them all day.
2. Read the three numbers — inside the left temple arm, near the hinge, or measure them if the print has worn off.
3. Hold the bridge closest — stay within about 2 mm of your reference bridge width. This is the dimension least worth experimenting with.
4. Hold the temple close — within about 5 mm of your reference temple length. Many ranges only offer 140 mm and 145 mm, so pick the nearer one.
5. Move lens width freely — this is where you change the look. A 4 mm to 6 mm step up reads as noticeably larger.
6. Check the listing states all three — if a retailer only publishes lens width, you cannot size from it. Ask, or find the frame's spec on the brand's own site.

Metal frames with adjustable nose pads give you some tolerance on bridge width, because a technician can bend the pad arms. Acetate frames with a moulded bridge give you almost none, so the printed number matters more.

Why do two frames with the same numbers feel different?

Because the three numbers do not describe everything. Lens height is rarely printed but changes how much of the cheek a frame covers. Face form, the horizontal wrap of the front, changes how close the lenses sit. Temple design decides whether the arm grips behind the ear or rests straight along the head.

Manufacturing tolerance also applies to eyewear, as it does to clothing. Published garment specs carry a tolerance of roughly ±0.3" to ±0.8" depending on the fabric and process, which is why ShirtMath never treats a single published figure as exact. Frames are moulded and assembled rather than cut and sewn, so their tolerances are different, but the principle is the same: a printed number is a target the factory aims at, not a guarantee of the piece in the box.

Why do letter sizes not work here?

Eyewear escaped the problem clothing has. Frames are sold by dimension rather than by S, M and L, so the number on the frame means the same thing at every brand. Clothing is not like that.

The published specifications ShirtMath holds show what happens when a letter carries the label instead of a measurement. Retrieved 27 July 2026 from each brand's own spec sheet, a unisex Gildan G500 Medium publishes a 20.0" flat chest and a Next Level 3600 Medium publishes 20.5". A ladies' Gildan G500L Medium publishes 17.5". That is a 3 inch gap on the same letter, and the gap does not close at Large.

A frame stamped 18 has an 18 mm bridge whoever made it. A shirt labelled Medium does not have a fixed chest, which is the whole subject of vanity sizing. Sunglasses solved this by printing the measurement on the product.

Can ShirtMath measure sunglasses from a photo?

No. ShirtMath's scanner photographs a shirt laid flat next to a scale card and reads its flat measurements, then matches those against published garment measurements across the catalogue. Sleeve opening and bicep are the least reliable readings, and anything the scanner cannot read confidently is left blank rather than guessed. The methodology page sets out what the scanner reads and where it stops.

It does not measure sunglasses from a photograph. It does not analyse face shape, and it will not. A photograph on its own has no reliable scale reference on a face, which is why the three numbers printed inside your existing frame are worth more than any image-based estimate. This page is reference material for reading those numbers.