The sunglasses size calculator above does something a size chart on its own cannot: it converts the three numbers printed inside a frame into a single width you can compare against your own face, and tells you how far apart they are. Those three numbers are the only objective description of a frame's fit that manufacturers reliably publish, and almost nobody knows how to read them.
Arb Digital builds free calculators that source every figure they present rather than inventing a chart that looks plausible. That matters more in sizing than almost anywhere else, because a made-up table produces confident advice about a purchase, and every band in the chart used on this page comes from a named eyewear maker's own published size guide, linked below.
What the Three Numbers on a Frame Mean
Look inside the temple arm of any pair of glasses or sunglasses and you will find three figures, usually written as 52-18-145, sometimes as 52 □ 18 145 with the small square standing for the bridge. All three are millimetres, and they always appear in the same order.
The first is lens width, also called eye size: the horizontal width of one lens at its widest point. The second is bridge width, the gap between the two lenses that sits over the nose. The third is temple length, measured along the arm from the hinge to the tip, including the bend that goes behind the ear.
Randolph's guide on how to find your sunglasses size sets out the same three measurements and makes a point worth repeating: the most reliable starting point is a pair you already own and like, because the numbers inside them describe a fit you have already tested on your own face.
How to Use It
- Measure your face width across the front at the widest point of your temples, just above the ears, in millimetres. A ruler held against a mirror works, though a second person gets a straighter line.
- Enter the three numbers from the frame you are considering. If you are shopping online, they are almost always in the product specification.
- Adjust the end piece allowance if you know it. Six millimetres a side is a reasonable default for a typical frame, and setting it to zero compares only the marked span.
- Set how far from your face width you are willing to be. Five millimetres either way is a sensible starting window.
- Read the difference and the suggested lens width, which is the lens size that would put a frame with your chosen bridge at exactly your face width.
The Formula and How It Is Calculated
The marked span across the front of a frame is simply two lenses plus the bridge between them:
Marked span = (2 × lens width) + bridge width
The real frame front is wider than that, because the end pieces — the corners carrying the hinges — sit outside the lenses and are not covered by any of the three published numbers. Adding an allowance for each side gives the estimated total:
Total frame width = (2 × lens width) + bridge width + (2 × end piece allowance)
Work through the defaults. A 52-18-145 frame has a marked span of (2 × 52) + 18 = 122 mm. With a six-millimetre end piece each side, the estimated total front width is 134 mm. Against a 140 mm face width, that is six millimetres narrower than the face — just outside a five-millimetre window, so this frame reads as slightly narrow.
Running it backwards gives the lens width that would match exactly. Rearranged, lens = (face width − bridge − 2 × end piece) ÷ 2, which here is (140 − 18 − 12) ÷ 2 = 55 mm. So on a 140 mm face with an 18 mm bridge, a 55 mm lens would sit flush to the temples.
One caveat on the end piece allowance. It is an estimate, not a published specification — brands almost never state it, and it varies with frame construction, from a slim metal corner to a thick acetate one. That is why it is an editable field here rather than a hidden constant, and why the marked span is reported separately so you can always see the number that came straight off the frame.
Where the Size Bands Come From
The size classification in the fourth grid item is not ours. It comes from the published size guide of a single named maker, Cutler and Gross, whose size guide for opticals and sunglasses sets out four separate bandings in millimetres: lens width small 40–48, medium 49–54 and large 55–62; bridge narrow 14–19, medium 20–22 and wide 23–24; temple length regular 145, long 150 and extra long 155; and frame width narrow 130–140, medium 140–150 and wide 150–155.
Using one maker's chart rather than a blended average is a deliberate choice. Averaging several brands' charts produces a table that describes no actual product, and inventing bands that look reasonable would be worse still. This one is attributable, checkable and dated to the page it came from.
The consequence is that the bands describe how that maker classifies its own range. Another brand's "medium" may sit somewhere different, and its frame front may be built to a different proportion entirely. The bands are useful for orientation — is this a small, medium or large frame in ordinary terms — and not as a rule any brand is obliged to follow.
The default 52-18-145 frame in this calculator lands as a medium lens, a narrow bridge, a regular temple and, at an estimated 134 mm front, a narrow frame width. That combination is entirely ordinary, which is rather the point: most frames sit in the middle of these ranges, and the interesting information is the comparison against your own face, not the label.
Why Frame Width Is the Measurement That Matters
Of the numbers available, total frame front width relative to your face width is the one that predicts whether a pair will sit properly. A frame much narrower than the face pushes the temple arms outward, which puts pressure just in front of the ears and makes the arms splay; a frame much wider slides down the nose and gives a floating, disconnected look, with sunglasses letting light in at the sides.
Lens width alone does not tell you this, because it interacts with the bridge. A 50 mm lens with a 22 mm bridge spans 122 mm, exactly the same as a 52 mm lens with an 18 mm bridge. The two frames occupy the same width on your face while looking quite different and sitting quite differently on the nose.
That is why bridge width is the second thing to get right rather than an afterthought. It controls how high the frame sits and where the weight rests. Cutler and Gross also notes a useful visual check that no calculator can perform: your pupils should sit in the centre of the lenses. If they sit noticeably inward or outward, the bridge is wrong for you even when the total width looks correct.
Temple length is the most forgiving of the three. It affects how far back the bend falls relative to your ear, and it is the measurement most often adjusted by an optician after purchase, since acetate temples can be heated and reshaped and metal ones bent.
What a Measurement Cannot Tell You
Be clear about the limits. A frame is a three-dimensional object being judged here by one horizontal number. Frame curvature — the wrap around the face, which is often pronounced on sports and driving sunglasses — changes how a given front width sits without changing any of the three published figures. So does the pantoscopic tilt of the fronts and the shape of the nose pads.
Nor does any of this address the shape question. Whether a round or a square frame suits you is a styling convention rather than a measurement, and our face shape calculator covers that side descriptively. It is a separate question from fit, and the two are frequently confused.
The honest framing is the one every optician gives: a measurement narrows the field of frames worth trying, and trying them is what decides. Brands differ, individual models differ within a brand, and a maker's own current chart for the specific frame outranks any general guidance including this page's.
If you are working through several body measurements at once, our hat size calculator handles head circumference, the glove size calculator covers hand span, and the ring size converter and shoe size converter move between national systems. All of them share the same caveat: the maker's chart wins.
Practical Notes on Measuring
Face width is easy to get slightly wrong. Measure across the front of the face rather than around the curve of the head, at the widest point of the temples just above the ear, and keep the ruler level. Measuring around the head instead of across it can add ten millimetres or more, which is enough to send you a full size out.
If you already own a pair that fits well, there is a shortcut that beats all of this: read the numbers inside them and use those as your reference. Enter that frame here first to see what total width it implies, then compare candidates against it. A fit you have already lived with is better evidence than any measurement of your face.
For general unit work, the length converter handles millimetres to inches, and the percentage difference calculator is useful if you want to express the gap between two frames proportionally rather than in millimetres.
Arb Digital designs and builds free interactive calculators that source every figure they publish and earn links because they are genuinely useful. Browse what we have already published, or tell us what your audience keeps searching for.
Browse the Free Tools Hub Talk to Arb DigitalCommon Mistakes to Avoid
- Measuring around the head instead of across the face. Face width is a straight line across the front at the temples. Following the curve can add ten millimetres or more.
- Judging a frame by lens width alone. The bridge is part of the span. A 50 with a 22 bridge is exactly as wide as a 52 with an 18.
- Assuming the marked span is the whole frame. The end pieces carrying the hinges sit outside the lenses and are not in any of the three published numbers.
- Treating one brand's size bands as universal. The bands here are Cutler and Gross's published chart for its own range. Other brands classify differently.
- Confusing fit with style. Width is a measurement; whether a shape suits you is a styling convention and a separate question entirely.
Related Free Tools From Arb Digital
For the styling side rather than the fit, use the face shape calculator. For other body measurements, the hat size calculator, glove size calculator, ring size converter and shoe size converter all follow the same sourced approach. The length converter and percentage difference calculator cover the general arithmetic. Everything else is on the free online tools hub.
Frequently Asked Questions
They are lens width, bridge width and temple length, always in that order and always in millimetres. A frame marked 52-18-145 has 52 mm lenses, an 18 mm bridge and 145 mm arms. They are printed inside a temple arm or behind the bridge.
Measure straight across the front of your face at the widest point of your temples, just above the ears, in millimetres. Keep the ruler level and do not follow the curve of the head, because measuring around rather than across can add ten millimetres or more.
The marked span is two lenses plus the bridge, so a 52-18 frame spans 122 mm. The real front is wider, because the end pieces holding the hinges sit outside the lenses and are not included in the published numbers. This calculator adds an adjustable allowance for them.
From Cutler and Gross's published size guide, which gives lens width bands of 40 to 48 small, 49 to 54 medium and 55 to 62 large; bridge bands of 14 to 19 narrow, 20 to 22 medium and 23 to 24 wide; temple lengths of 145 regular, 150 long and 155 extra long; and frame widths of 130 to 140 narrow, 140 to 150 medium and 150 to 155 wide.
Close to it is the usual guidance. A frame much narrower pushes the arms outward and presses in front of the ears; one much wider slides down the nose and lets light in at the sides. This page lets you set your own acceptable window rather than imposing one.
Because it is part of the total span and it controls where the frame sits. Two frames with the same overall width but different bridges rest differently on the nose and place your pupils differently in the lenses, which is the visual check an optician uses.
No. The three millimetre measurements are consistent because they are physical dimensions, but size labels such as small, medium and large are set by each brand for its own range. Always check the maker's own chart for the specific frame.
This page compares measurements you enter and reproduces a published size chart from a named maker. It is a starting point for choosing frames to try on, not a guarantee of fit, and it is not a substitute for advice from a qualified optician.
