What the numbers on sunglasses mean

The three numbers inside a sunglasses temple arm are millimetre measurements of the frame, always in the same order: lens width, bridge width, temple length. A frame stamped 52-18-140 has lenses 52 mm across, a bridge of 18 mm and arms 140 mm long.

Each figure is a physical dimension of the object. None of the three is a size grade like small or large, and none of them is a statement about a face. This page decodes the stamp, gives the range each number usually falls in, and explains what to do when the marking has worn away.

Most frames carry the stamp on the inner face of a temple arm, somewhere between the hinge and the tip. Check these places in order.

1. Inside the left arm — the usual location. Open the frame, turn the arm so its inner face catches a lamp, and look near the hinge.
2. Inside the right arm — many brands split the markings, putting the model name and colour code on one arm and the size trio on the other.
3. The back of the bridge — on thin metal frames there is no flat area on the arm, so the numbers are engraved above the nose pads instead.
4. The inner face of the front — some acetate frames stamp the size behind the nose area, where the arms cover it when folded.
5. Near the arm tip — a few makers print the temple length on its own, well away from the other two numbers, so a missing third figure is not always a missing figure.

The characters are small, often only about 1 mm to 2 mm tall, and are usually moulded or laser-etched rather than inked. Tilting the frame so light rakes across the surface raises a shadow in the letterforms, which makes a stamp readable that shows nothing when viewed straight on.

The first number is lens width: the horizontal width of one lens in millimetres. On a frame marked 52-18-140 it is 52 mm, and it describes one lens, not the pair.

It is taken under the boxing system, which draws the smallest rectangle that will contain the lens shape and measures that rectangle rather than tracing the outline. The distinction matters on shapes that are not rectangular. On a round or teardrop lens the boxed width is slightly wider than the widest span of glass you can actually see, because the box touches the shape at only a couple of points.

Lens width drives most of the width of the frame front. Two lenses sit either side of one bridge, so the front measures at least twice the lens width plus the bridge, and the end pieces at the hinges add more on each side. A 2 mm step up in lens width widens the front by at least 4 mm before the bridge and end pieces are counted.

The second number is bridge width: the horizontal gap between the two lenses at their closest point, in millimetres. It is sometimes labelled DBL, for distance between lenses. On a 52-18-140 frame it is 18 mm.

Bridge width moves the two lenses apart without changing the size of either one. Two frames with identical 52 mm lenses and bridges of 16 mm and 20 mm differ by 4 mm across the front, and the lenses sit 4 mm further apart on the wider one.

On a frame with adjustable metal nose pads the printed bridge figure tells you less than it does on a moulded acetate frame, because the pads can be bent in or out after purchase. The number still describes the frame as it left the factory.

The third number is temple length: the total length of the arm in millimetres, measured from the hinge screw to the end of the tip, following the arm around its bend rather than in a straight line. On a 52-18-140 frame it is 140 mm.

Temple length is the figure most often quoted in round steps. Adult frames are commonly marked 135, 140, 145 or 150, and a maker will frequently offer one model in only two of those. Because the measurement follows the bend, two arms with the same stamped number can still place their bend in different spots along the length.

These ranges describe what frame makers commonly publish for adult sunglasses. They are a guide to what is normal, not a specification any frame has to meet, and children's and oversized frames fall outside them on purpose.

Read the trio together rather than one at a time. A 58-14 front and a 54-18 front are the same total width across the lenses and bridge, but the glass and the gap are divided up differently, so the two frames are not interchangeable even though one addition matches.

Many frames print the stamp as a square symbol between the first and second numbers instead of a hyphen. The square is the boxing system mark. It states that the lens width was taken as the width of the smallest rectangle containing the lens shape, the convention set out in ISO 8624.

A hyphen, a square and a plain space between the numbers carry the same meaning in ordinary use. Where the order is ambiguous, the sizes themselves settle it: the temple length is far larger than the other two, and the bridge is the smallest of the three, so a 140 is never a bridge and an 18 is never an arm.

The size trio usually shares the arm with several other markings. Knowing which is which stops you copying a model code into a size field.

1. Model code — a letter and number reference such as a style number, often longer than the size figures and printed first.
2. Colour code — a two to four character code for the finish, sometimes separated from the model code by a slash or a space.
3. Filter category — a single digit from 0 to 4 describing how dark the tint is, marked on frames sold under the EN ISO 12312-1 labelling convention, usually on the arm or the swing tag.
4. Compliance marks — CE, UKCA or similar marks, which are letters rather than measurements.
5. Country of manufacture — a short word or abbreviation, generally at the end of the line.

If a line of characters is not two or three plain numbers in the ranges above, it is almost certainly not the size.

Moulded stamps wear off with handling, and etched ones fade first at the point where the arm rubs a case. Work through these in order, cheapest first.

1. Rake the light across it — hold the inner face at a shallow angle under a bright lamp so the remaining depressions throw a shadow.
2. Photograph and enlarge — a phone photo zoomed in on screen resolves faint characters better than a naked eye at reading distance.
3. Check the other arm and the bridge — the size may have been split across two locations, and the surviving half narrows the search.
4. Find the model code instead — a legible model and colour code lets you look the published size up on the maker's own product page, which is the most reliable answer available.
5. Check the case, box or receipt — the size trio is frequently printed on the swing tag, the inner label of the case or the order confirmation.
6. Measure the frame yourself — with a millimetre rule you can reproduce all three figures directly. The method is set out in how to measure sunglasses.
7. Ask an optician — a dispensing optician has a frame rule and reads these numbers daily, and most will do it in a couple of minutes.

Each figure moves one thing and leaves the others alone, which is what makes the trio worth reading rather than skimming.

1. Lens width — changes the size of the glass and, through it, the overall width of the front. It is the only one of the three that changes how much area the lens covers.
2. Bridge width — changes the spacing between the lenses and the total front width, and leaves the lens size untouched.
3. Temple length — changes how far back the arm reaches before its bend, and has no effect on the front of the frame at all.

That independence is why replacing a frame by matching only one number goes wrong. Two models sharing a 52 mm lens can differ by 6 mm at the bridge and 15 mm at the temple, which are different objects wearing the same first number.

The unit does. A 52 is 52 mm of boxed lens width whoever made the frame, because it is a physical measurement of the product rather than a label chosen by the seller. Small differences still creep in from where a maker places the box on an unusual lens shape and from production tolerance, so treat two frames marked 52 as close rather than identical.

This is the opposite of how letter sizing works in clothing. A medium shirt is a name, not a measurement, and the same letter is attached to garments of different widths by different brands and by the same brand across cuts. Published blank shirt specifications held by ShirtMath, retrieved 27 July 2026, show a ladies-cut Gildan G500L medium at a 17.5 inch flat chest against 20.0 to 20.5 inches for unisex mediums, a three inch spread on one letter. That gap is the subject of vanity sizing, and eyewear largely avoids it by printing millimetres on the product.

No, and that boundary is worth stating plainly. ShirtMath does not measure sunglasses from a photograph, does not identify frames from an image and does not analyse faces. This page is reference material, not a product feature.

What is live is shirt measurement. Photograph a shirt laid flat with a scale card and ShirtMath reads its flat measurements and matches them against published garment measurements across the catalogue. Roughly half of scanner readings land within an inch of a tape measure, sleeve opening and bicep are the least reliable of the readings, and anything the scanner cannot read confidently is left blank rather than guessed. The same honesty applies here: for frame numbers, the stamp and a millimetre rule are the tools, and a photograph is not one of them.