Sunglasses Fit: Lens Width, Bridge and Temple in Millimetres

The three small numbers printed inside a sunglasses arm are the only reliable way to compare frames you cannot try on. Read left to right they are lens width, bridge width and temple length, so 52-18-140 means a 52 mm lens, an 18 mm nose gap and 140 mm arms. Two frames with identical lens widths can fit completely differently, because the other two numbers decide where the frame actually lands on your face.

Reading the code inside the arm

Look on the inside of one temple arm. The three figures usually sit beside a model code and a colour code, separated by dashes or a small square. Adult frames typically run about 48 to 62 mm for lens width, 14 to 24 mm for bridge and 120 to 150 mm for temple length.

A pair you already own is the best reference: match its three numbers. That pair encodes a fit test that no questionnaire replicates. If the current pair slides down your nose, the bridge figure it carries is too wide for you; if it pinches, too narrow. Note the number and the complaint together and you have a target better than any face-shape chart.

Two practical notes on finding the code. The stamping sits under paint on acetate arms and can wear away on older frames; the sticker on the original lens, the box or the case card usually repeats it. And check that the code belongs to the exact model variant: many frames ship in more than one size, and the same model name can carry a 50 mm version and a 55 mm version a season apart. A code copied from a different variant is worse than no code, because it borrows credibility it has not earned.

Measure your face before the frame

Face a mirror and measure straight across from one temple to the other, just above the ears. That figure is close to your ideal total frame width. Up to about 130 mm suits narrower faces and usually pairs with 47 to 50 mm lenses; 131 to 138 mm is where most adults land, with 51 to 54 mm lenses; 139 mm and above suits wider faces, with 55 to 60 mm lenses. A payment card is 85.6 mm wide if you need a scale; hat sizing uses the same tape around the head.

Measuring an existing frame takes a minute and cross-checks the face measurement. Lay the frame closed on a table, measure across the front at the hinges, and compare that to the total width your face measurement implied. The two should land within a few millimetres of each other. Where they disagree, trust the frame you actually wear without noticing it, because unconscious comfort is the measurement that matters.

The arithmetic that turns three numbers into a width

The code also adds up. Total frame width is roughly twice the lens width plus the bridge, plus a few millimetres of rim and hinge on each side. A 52-18-140 frame therefore spreads about 128 mm across the front before the hinges, which lands it in the middle band of the face measurement above. Do this sum before buying and the listing’s “oversized” claim becomes checkable: a 58 mm lens with an 18 mm bridge is already over 140 mm of front before any hardware, and no marketing term changes that.

The same arithmetic runs in reverse for a frame you love but that sits wrong. If the front is a millimetre or two too wide, the bridge is usually the culprit, not the lenses, because rim thickness varies less between models than bridge shape does. That points the fix at the right number instead of a different style altogether.

Why lens width alone misleads

Bridge width decides where the frame sits on the nose, and it is the number people skip. The right lens width with too wide a bridge slides down; too narrow a bridge pinches and leaves marks. Temple length decides whether the arms reach the ear without pressing, and the bend should start roughly where the ear starts. Change the bridge before shrinking the frame — the split watch case sizes describes.

The nose interface comes in two constructions, and they fail differently. Fixed acetate saddles carry the frame on the bridge of the nose and forgive nothing: the bridge figure has to be right, because there is no adjustment. Separate nose pads on metal arms adjust outward or inward and spread the load over a smaller area, which suits lower nose bridges where a saddle tends to rest on skin instead of bone. A frame that “has no nose pads” is therefore not a style note; it is a warning that the bridge number is non-negotiable.

UV400 is not polarisation

UV400 means the lens blocks wavelengths up to 400 nanometres, covering both UVA and UVB; a label reading 100% UVA and UVB protection means the same. Lens darkness has nothing to do with it, and a dark tint without UV filtering is worse than bare eyes: the pupil dilates behind it.

Polarisation is a separate filter that cuts glare off water and off roads; off snow too. It is not UV protection, and the two are checked separately.

The labels are also easy to cross-contaminate when a retailer paraphrases them. A listing that says “polarised UV400” is making two claims, and a listing that says only “polarised” has made one. When the two appear joined in a product title but the specification table lists neither, treat the specification table as the truth and the title as marketing shorthand; the reverse habit is how dark tints with no filtering get bought as sun protection.

The practical consequence is that a comfortable-fitting frame with the wrong lens claim is still the wrong frame. Fit decides whether you wear a pair all day; the UV claim decides what your eyes receive while you do. Both belong on the pre-purchase checklist, and only one of them is printed inside the arm.

Temple length and the bend behind the ear

Temple length runs from the hinge to the tip of the arm, following the curve, and it is the third number for a reason. If the bend starts well before the ear, the arms press on the skull and the frame gets pushed forward and down; if the arm reaches past the ear before bending, the tips float and the frame’s weight hangs off the nose alone. The ear-start rule in the section above is the check: the downward bend should begin at roughly the top of the ear, then follow the skull to the tip.

Temples are also where adjustment does the most good per minute spent. An optician can bend a metal or acetate arm outward for a wide face or inward for a narrow one in minutes, and hooking the tips down slightly keeps a slightly short arm from sliding. This is the one fit variable that can be tuned after purchase on most materials — remember that TR90, from the materials section above, is the exception.

What sliding, pinching and marks each tell you

Fit complaints map to numbers, and the mapping is worth memorising before a purchase. A frame that slides is usually a bridge too wide, sometimes combined with front-heavy materials; the fix is a narrower bridge figure, not tighter temples. A frame that pinches at the nose is a bridge too narrow, and no amount of temple bending relieves it. Red marks after an hour mean the frame’s weight is landing on too little nose area, which points at nose-pad construction as much as at the bridge figure.

Pressure above the ears is the one symptom that does belong to the temples. If the arms grip the skull rather than resting, temple spread or length is the number to change. Diagnosing one symptom at a time matters because the fixes conflict: adding nose-pad width to cure a mark will start a slide if the bridge was right all along. Read the symptom, change one number, re-test.

What frame materials change

Acetate is the classic plastic, roughly 20 to 30 g a frame and heat-adjustable. Titanium is the lightest metal at about 8 to 18 g, corrosion-resistant but stiffer to adjust. TR90 is an injection-moulded nylon around 9 to 14 g, flexible and impact-resistant, and cannot be heat-adjusted. Stainless steel runs about 15 to 30 g: rigid, good at holding an adjustment, and usually contains nickel.

Weight and adjustability are fit properties, not spec trivia. A heat-adjustable acetate or metal arm can be re-bent at the ear by an optician to fix a temple-length problem; a TR90 frame cannot, so its stock geometry is permanent. Weight matters more as lens size grows: a 60 mm lens in steel pushes toward the top of the gram range, and front-heavy frames slide before the bridge is even wrong. Nickel content matters to anyone with the metal allergy, which is common enough that titanium exists as a category.

Three frames, three codes

Three frames show the range:

• Ray-Ban Original Wayfarer RB2140
• Oakley Holbrook OO9102
• Maui Jim Peahi

The Wayfarer is narrow with the widest bridge, the Holbrook opens the wide band at 55 mm, and the Peahi is widest.

Run the sum on each and the spread becomes visible. The Wayfarer’s 50 mm lens era sits under the mid-band despite its fame as a big frame, which is why small faces wear it and wide faces find it pinching at the temples. The Holbrook and the Peahi climb from the top of the standard band into genuine wide territory. All three are acetate with fixed saddles, so for each of them the bridge figure is the whole nose-fit negotiation — there is no pad to bend. A buyer between bands should treat the lens-width step as the decision and let the famous names follow, not lead.