The numbers stamped inside a temple arm — something like 52□18-140 — describe the frame’s physical dimensions, not the prescription. The first figure is lens width, the second is bridge width, and the last is temple (arm) length, all in millimetres. They tell you how the frame is built, not how well you’ll see through it.
Where are these numbers printed on a frame?
Look on the inside of one of the temple arms, or sometimes on the inner edge of the bridge. Most manufacturers stamp it in small print, often alongside a model name or colour code. It’s the same convention across brands and price points, which is why it’s worth learning once.
A typical string reads 52□18-140. The square (□) is a formatting convention, not a measurement — it simply separates lens width from bridge width and signals that both numbers relate to the frame’s front. Some frames also print a model number and a colour code (often four digits) nearby; those are internal references for reordering the same frame, not sizing information.
What does the first number mean?
The first number is the lens width — the horizontal measurement of one lens, taken at its widest point, in millimetres. Most adult frames fall somewhere between the low 40s and low 60s. It’s the single figure that most affects whether a frame reads as delicate or bold on a given face.
A 44mm lens on a narrow face can look considered and precise. The same 44mm on a wider face can look pinched, as though the frame has been borrowed from someone smaller. There’s no universal “correct” number — it’s a proportion question, and proportion is relative to the rest of the face, not an absolute target.
What does the second number mean?
The second number is bridge width — the gap between the lenses, measured at the narrowest point, again in millimetres. It’s usually the smallest number in the sequence, often somewhere between 14mm and 22mm, and it’s the figure most people overlook when a frame doesn’t sit right.
Bridge width is doing more work than it gets credit for. Get it wrong and the frame slides down the nose, sits crooked, or presses uncomfortably at the sides — regardless of how well the lens width and arm length suit the face. A frame that looks right in a mirror but keeps drooping mid-conversation is very often a bridge-width problem, not a face-shape problem.
What does the third number mean?
The third number is temple length — the arm’s total length from hinge to tip, measured in millimetres, typically between 135mm and 150mm. It determines how the frame sits over the ears, and whether it grips comfortably or presses at the temples by the end of a day.
Standard lengths cover most adult heads, but ears and head width vary more than people expect. An arm that’s technically “the right size” for the lens width in front of it can still be the wrong length for a particular person — which is one reason two people can try the exact same frame and get two very different fits.
Do these numbers tell you anything about the prescription?
No. These three measurements describe the frame’s construction only — how wide the lenses are, how far apart, and how long the arms run. Nothing about lens power, prism or how a prescription is applied to the lens is stamped inside the frame, and reading a prescription is genuinely a job for an optician, not a style publication. If you’re holding a prescription and trying to work out what its figures mean for you, that conversation belongs with the person who wrote it.
Why do frame sizes vary so much between brands?
Because there’s no single universal head, and no legal requirement to standardise proportions across brands. A “medium” from one house can measure closer to a “small” from another. The numbers inside the frame are the only reliable way to compare like with like, since size labels themselves aren’t consistent.
This is precisely why the internal measurements matter more than the marketing size on a display card. Two frames labelled “medium” might differ by several millimetres in lens width alone — enough to change how a frame reads on the same face. When comparing options, it’s worth thinking about how frame proportions interact with face shape rather than trusting a size label at face value.
Does a bigger lens number mean a better fit?
Not automatically. Lens width only tells you the frame’s horizontal reach — it says nothing about depth, how the frame sits on the bridge of the nose, or how the shape relates to brow line and cheekbone. A larger number can be entirely wrong for a face, and a smaller one entirely right.
As a general (and genuinely optional) styling convention: narrower faces often carry smaller, more angular lens shapes well, while broader faces can hold larger or rounder shapes without them reading as oversized. These are starting points, not rules — plenty of people look better deliberately working against them, choosing an oversized frame on a narrow face for effect, or a small, precise shape on a broad one for restraint. Anyone building intuition for what suits them is better served spending time with the numbers on frames they’ve actually tried in front of a mirror, using our notes on how frame fit is actually judged as a reference point rather than a rulebook.
How do these numbers relate to how a frame actually feels?
They’re a starting point, not a guarantee. Two frames with identical stamped measurements can still feel different depending on hinge quality, temple curve, and how the frame is weighted front to back. The numbers narrow the search; they don’t finish it.
This is particularly true of material. A frame in a lightweight acetate or titanium can carry a larger lens width comfortably, where the same dimensions in a heavier material might feel front-loaded on the nose. If a frame’s numbers look promising on paper but the fit still feels off in person, material and construction are usually where the answer lies — not the millimetres.
What about the numbers on sunglasses?
Sunglasses are stamped the same way — lens width, bridge width, temple length — because the frame construction is measured identically whether or not the lenses are tinted. The tint itself carries separate information that matters far more for protection than any of the size numbers.
Whatever the depth of tint, a genuinely protective lens will state a UV rating — look for UV400 or 100% UVA/UVB protection, ideally referencing a recognised standard. A dark lens without that rating is worse than wearing no sunglasses at all, because the tint causes the pupil to open wider while offering no actual filter, letting more UV reach the eye than it would unprotected. Darkness is a cosmetic property of the lens; UV protection is a separate, stated fact, and it’s worth checking before assuming any pair of sunglasses is doing its job.
Do the numbers change if I ask for a different lens type?
The frame’s stamped measurements stay the same regardless of which lenses go into it — they describe the frame, not the glass or plastic sitting inside it. Lens choices, including tint, coatings, or filtering claims, are a separate conversation from frame sizing.
One area worth a note of caution: lenses marketed as blue-light filtering for screen use are increasingly common, but the evidence that they meaningfully reduce eye strain or protect vision from screen use is thin and unsettled. That doesn’t make them pointless as a personal preference — some people simply prefer how a slight tint looks or feels — but it’s not a claim worth taking at face value. For anything involving how a specific lens type or prescription should be applied, that decision sits with an optician, not with frame markings or marketing copy.
Is it worth checking these numbers before buying online?
Yes — it’s the most reliable way to compare a frame you can’t try on against one you already own and like. If a current pair fits well, its stamped numbers become a useful benchmark for evaluating anything new.
Before buying glasses without trying them on, measure a well-fitting pair at home and compare the three figures against the listing. It won’t guarantee an identical feel — material and hinge construction still vary — but it removes most of the guesswork around whether a frame will sit anywhere near correctly on a first wear.
Should these numbers affect how I care for a frame?
Indirectly. Bridge width and temple length affect how much stress a frame takes on daily wear — a tight bridge that’s constantly adjusted, or arms slightly too short and under tension, will loosen hinges and wear finishes faster over time.
Frames sized correctly from the start tend to need less handling, which means less contact, fewer adjustments, and generally a longer working life. It’s one more reason the numbers inside the temple are worth reading properly, not just glancing past — the details covered in general glasses care guidance work best on a frame that fits well to begin with.
A final word on urgent eye symptoms
None of this — frame numbers, lens tint, or fit — has any bearing on sudden changes in vision. A sudden change in sight, eye pain, flashes of light, new floaters, halos around lights, or vision loss in one eye are same-day matters for a professional, not something to wait out or self-assess. If any of that applies, the appropriate next step is an eye care professional, not a mirror or a fitting guide.
The Designer Glasses publishes general information and style opinion, not professional advice. We are writers and enthusiasts, not opticians, optometrists or dispensing opticians. Nothing here interprets a prescription, recommends a lens power, or tells you whether you need correction — only an eye examination can do that. If your vision changes suddenly, or you have eye pain, flashes, new floaters or halos around lights, treat it as urgent and contact an eye care professional the same day. Sunglasses should carry a stated UV rating: a dark lens without UV filtering is worse than none at all.
Related reading
Choosing Glasses, How Eyewear Works, What Is a “Forever” Frame, and Does It Exist?, Why Do Some Frame Shapes Date Faster Than Others?, Why Do Round Frames Read as Intellectual? A Style Guide.