Sunglasses Sizing: How to Read the Numbers and Get the Fit Right

Almost every complaint about sunglasses – they slide down, they pinch, they leave marks, they sit crooked – is a sizing problem rather than a quality problem. And sizing is one of the few things in eyewear that is genuinely objective: there are three numbers printed on the frame, and once you know yours, the guesswork ends.

Reading the numbers on the frame

Look on the inside of the left temple arm, sometimes on the bridge. You will find something like 52-18-140, occasionally with a small square between the first two figures.

• Lens width – the horizontal width of one lens in millimetres. Typically 48 to 62.
• Bridge width – the gap between the lenses, where the frame crosses your nose. Typically 14 to 24.
• Temple length – the arm, from hinge to tip, including the bend. Typically 135 to 150.

Those three figures describe the frame completely enough to buy online with confidence, provided you know what yours should be.

Find your numbers from a pair that fits

The fastest method is not to measure your face. It is to measure a pair you already get on with.

Find the marking inside the temple of the frames that feel right and write the three numbers down. That is your baseline, and staying within a couple of millimetres of it will get you a comfortable fit far more reliably than any face-shape rule.

If no marking is present, measure the lens at its widest point and the gap between the lenses at the narrowest, both in millimetres, with a ruler.

Starting from scratch

With no reference pair, measure the width of your face at temple height – the distance across from one temple to the other, just in front of the ears.

Most adult faces fall between roughly 128mm and 150mm. Total frame width should sit close to that figure. A frame more than a few millimetres wider will slide down; a frame narrower will press against the sides of your head and leave marks within the hour.

Total frame width is roughly twice the lens width, plus the bridge, plus the frame material at each edge – so a 52-18 frame lands somewhere near 130mm depending on the rim.

Bridge width decides whether they stay put

This is the measurement that matters most and gets the least attention.

Too wide and the frame rests on your cheeks and slides with every expression. Too narrow and it perches high, pinches, and leaves indentations either side of the nose.

Bridge style matters as much as the number. Keyhole bridges sit on the sides of the nose and suit narrower or lower-set noses. Saddle bridges spread weight across a wider area and suit heavier frames. Adjustable nose pads, common on metal frames, are the most forgiving option of all – they let an optician tune the fit rather than requiring the frame to be right out of the box.

Temple length

The arm should run straight back to just past the top of your ear, then curve down without pressing.

Too short and the bend lands in front of the ear, pushing the frame forward off your face. Too long and there is nothing holding the back of the frame, so it slides at the slightest tilt.

Temple length is adjustable within a range on most frames – the curve can be reset warm on acetate, or bent on metal – so it is the measurement with the most tolerance. It is still worth being close.

Checking the fit on your face

• Your pupils should sit near the horizontal centre of each lens, not toward the inner edge.
• The frame should not touch your eyelashes, and should not rest on your cheeks when you smile.
• The outer edges should sit roughly level with the sides of your face – a little wider is a style choice, much narrower is a fit failure.
• The top rim should follow your brow line rather than cutting across it at an angle.

The ten-minute test

Before committing, wear the frame for a full ten minutes. Look down at a phone, tilt your head, smile, walk around.

A correct frame stays put through all of it and leaves no impression when you take it off. If you are aware of it at minute nine – a pressure point, a slide, a weight on the nose – the size is wrong, and no amount of liking how it looks will fix that. Small fit errors do not fade with familiarity; they become the reason a pair lives in a drawer.

Finally: a frame within a millimetre or two of correct can usually be adjusted to fit properly, free, by an optician. A frame badly outside your range cannot. Sizing gets you into the right territory – the bench does the rest.