How to measure sunglasses
Four measurements describe a sunglasses frame, all in millimetres: lens width, bridge width, temple length and total frame width. A millimetre ruler gives you all four, stamp or no stamp.
Last verified: 26 August 2026
What are the four numbers you need?
Four measurements describe a sunglasses frame, and all four are recorded in millimetres: lens width, bridge width, temple length and total frame width. A millimetre ruler gives you every one of them in about five minutes, with no size stamp required.
The first three are the figures a maker normally prints on the inside of the left temple arm, in that order. Total frame width is rarely printed, so it usually has to be measured whatever the arm says. The naming and the boxed measuring system behind these figures are set out in ISO 8624, the international standard covering spectacle frame measurement and terminology.
| Measurement | What it describes | Stamped on the arm? | Common range |
|---|---|---|---|
| Lens width | Widest horizontal span of one lens | Yes, first number | 40-62 mm |
| Bridge width | Shortest gap between the two lenses | Yes, second number | 14-24 mm |
| Temple length | Hinge screw to tip, following the bend | Yes, third number | 120-150 mm |
| Total frame width | Outer edge to outer edge across the front | Rarely | 125-150 mm |
| Lens height | Tallest vertical span of one lens | Rarely | 30-55 mm |
What do you need before you start?
A ruler marked in millimetres is the only essential tool. A steel rule is easier to read than a plastic school ruler because the scale starts closer to the edge. A digital caliper is better again for lens width and bridge, since it sits against the frame rather than hovering above it.
Temple length is the one measurement a rigid ruler handles badly. The arm bends down behind the ear, and a straight ruler measures the chord rather than the arm. Use a flexible tailor's tape, or lay a piece of string along the arm and then measure the string.
Work on a flat, hard surface with the frame in front of you, and write each figure down as you take it. Reading four numbers and then trying to recall them is how transcription errors happen.
How do you measure sunglasses, step by step?
Take the measurements in the order below. Each one is independent, so a mistake in one does not corrupt the others.
- Step 1: lay the frame front down — open both temple arms fully and rest the front of the frame on the table with the lenses facing down. This keeps the front flat instead of letting it rock on its curve, which is what makes horizontal readings drift.
- Step 2: measure lens width — measure one lens horizontally at its widest point, from the edge of the lens on the nose side to the edge on the outer side. The boxed system measures the lens, not the surrounding rim, so read to the inside of the frame rim on a full-rim pair. Record to the nearest millimetre. This is the first stamped number, sometimes called the eye size.
- Step 3: measure the bridge — measure the shortest horizontal gap between the two lenses, at the point where their nasal edges come closest together. Measure lens edge to lens edge and ignore the nose pads, which sit inside that gap and will give you a smaller figure. This is the second stamped number.
- Step 4: measure temple length — start at the centre of the hinge screw and follow the arm along its length, over the bend, to the very tip. Keep the tape in contact with the arm the whole way. This total, not the straight part alone, is the third stamped number, and it is normally quoted in five millimetre increments such as 135, 140, 145 or 150.
- Step 5: measure total frame width — with the frame still front down, measure straight across the front from the outermost point on one side to the outermost point on the other, usually at or just past the hinges. This number is not on the arm and it is the one that decides whether a frame spans your face or sits inside it.
- Step 6: measure again — repeat each of the four readings once. If the two readings for a measurement disagree by more than a millimetre, take a third and use the middle value.
Lens height is worth adding if you are comparing a pair you like against a listing. Measure the tallest vertical span of one lens, at right angles to the lens width reading. It is almost never stamped, and it is the measurement that most often explains why two frames with identical stamped numbers look nothing alike.
Why does my ruler reading not match the number stamped on the arm?
Because a stamp is a nominal specification for a model, not a measurement of the individual pair in your hand. Two things push the figures apart.
The first is rounding. Temple lengths are conventionally published in five millimetre steps, so an arm that genuinely measures 142 mm is very likely stamped 140. Lens width and bridge are usually given as whole millimetres, which hides anything below that.
The second is where you put the ruler. On a thick full-rim acetate frame, reading to the outside of the rim instead of the lens edge can add several millimetres to lens width. Bridge readings taken across the nose pads rather than the lens edges come out narrower. Neither is a mistake in the ruler; it is a mistake in the reference point.
A reading taken by eye against a millimetre scale is realistically good to about a millimetre either way, before any of that. Treat a one or two millimetre difference from the stamp as normal and a ten millimetre difference as a sign that you measured something other than what the stamp describes.
Where is the size stamped when it exists?
On most frames it is printed inside the left temple arm, near the hinge, in a sequence such as 52 □ 18 140. The small square between the first two numbers is the boxing symbol; it marks the figures as boxed system measurements rather than any other convention. Some makers put the sequence on the inside of the bridge instead, or split it between the bridge and the arm.
The stamp wears off. Printed markings on the inside of an arm sit exactly where fingers and hair rub against them every day, and on a pair you have owned for a few years there may be nothing legible left. That is the ordinary reason to measure rather than read, and it is why the ruler method above does not depend on the stamp at all.
If your stamp is intact and you want to know what each figure controls, what the numbers on sunglasses mean takes the sequence apart. For how those figures map onto the small, medium and large labels used in shops, see the sunglasses size guide.
What can these four numbers not tell you?
They describe the frame. They do not describe your face, and this page makes no claim about which frame suits which face. Total frame width can tell you that one frame is 8 mm wider across the front than another, which is a fact about two objects. Turning that into a recommendation would require a measurement of you, taken the same way, and a ruler laid on a table cannot supply it.
A photograph cannot supply it either. ShirtMath does not measure eyewear from an image, does not identify frames from a photo, and does no face-shape analysis of any kind. The four numbers above come from your own ruler, and that is the honest boundary of this page.
What ShirtMath does measure from a photograph is a shirt laid flat with a scale card in the frame, and it compares that 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 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.
Why does eyewear publish numbers when clothing does not?
Eyewear inherited a measuring system and prints it on the product. Clothing mostly prints a letter. That difference is the whole reason a page like this one is short and a page about shirt sizing is not.
Held against published blank specifications, a ladies' Gildan G500L Medium lists a 17.5" chest measured flat, while a unisex Gildan G500 Medium lists 20.0" and a Next Level 3600 Medium lists 20.5". That is a gap of up to three inches on garments carrying the same letter, and the letter gives you no way to see it coming. A frame stamped 52 □ 18 140 at least tells you what you are holding.
The same drift in clothing letters is covered in the guide to vanity sizing, which is where the sizing letter stops being a size and starts being a marketing decision.
Common questions
Do I need a caliper, or will a ruler do?
Can a photograph measure my sunglasses?
Which lens do I measure if the two lenses look different?
Is temple length measured straight, or around the bend?
Where to go next
Frames publish their numbers; shirts usually do not. Photograph a shirt laid flat with a scale card and ShirtMath will read its measurements and compare them against published specs across the catalogue.
Scan a shirt