Decode the numbers inside your glasses

Why these three numbers decide whether a frame works

Buying glasses online fails in one of two ways. Either the frame arrives and looks wrong on your face, or it looks fine and the lenses feel subtly off. Both failures are usually predictable from three numbers printed inside the frame before you order anything.

Those numbers are lens width, bridge width and temple length, always in millimetres, usually written as something like 52-18-140. They are stamped on nearly every frame made, which means you can read the fit of any pair you already own and use it as a reference for every pair you buy afterwards.

Lens width

The first number is the horizontal width of a single lens opening at its widest point, sometimes called the eye size. Common values run from about 44 mm on small frames to 58 mm or more on oversized ones, with most adult frames sitting between 48 and 54.

On its own, lens width tells you how large the frame will look. Combined with the bridge it tells you something far more useful.

Bridge width

The second number is the distance between the two lens openings, measured at their closest point. It typically runs from 14 to 22 mm. A bridge that is too narrow pinches and leaves marks; too wide and the frame slides down your nose no matter how the temples are adjusted.

Temple length

The third number is the full length of the arm, measured along its length including the curve that goes behind the ear. Common lengths are 135, 140, 145 and 150 mm, with 140 the most frequently made. This is the most forgiving of the three numbers, because temples can be heated and adjusted at the bend by any optician, usually for free.

The number that actually matters: frame PD

Add the lens width to the bridge width and you get the distance between the geometric centres of the two lens openings. That total is effectively the frame's own pupillary distance. A 52-18 frame has a frame PD of 70 mm.

Now compare that to your own PD. Suppose yours is 64 mm. The frame's centres are 6 mm further apart than your pupils, so each lens must be shifted inward by 3 mm to put its optical centre in front of your eye. That shift is called decentration, and it is the single most useful thing these numbers tell you.

What decentration costs you

Decentring a lens is routine. Every lab does it, on almost every job, and up to about three millimetres per lens it has no practical consequence at all. Beyond roughly five millimetres it begins to show, and how much it shows depends almost entirely on your prescription.

In a weak prescription, below about two dioptres in either direction, you can decentre a long way before anything is noticeable. In a strong one, the lens has to be ground from a blank that is offset relative to the frame opening, so one edge ends up considerably thicker than the other. In a minus lens that means a thick outer edge that catches the light; in a plus lens it means extra weight where you least want it. Push far enough and you introduce unwanted prism, which is the sensation of the image being shifted slightly — the thing that produces eye strain people usually blame on the prescription.

This is why a frame that looks perfect can still be the wrong frame, and why the arithmetic is worth doing before you spend the money rather than after.

Overall front width and your face

Two lens widths plus a bridge, plus roughly 12 mm for the endpieces and hinges, gives the total width of the frame front. For a 52-18 frame that is about 134 mm.

Compare it against the width of your own face measured temple to temple. A front significantly wider than your face will sit loose and slide down whatever the temples do. A front significantly narrower will press against the sides of your head, leave marks, and push the frame forward off your nose. Within about five millimetres either way, most people are comfortable.

Shopping by numbers instead of by picture

Once you know your PD and know what a good frame PD looks like for you, buying online stops being a gamble. Take the pair you own that fits best, add its first two numbers, and treat that total as your target. Then filter by lens width and bridge on any retailer's site and add the two numbers of anything that catches your eye.

This also explains something that puzzles people: two frames can both be marked 52 and fit completely differently, because one is 52-16 and the other 52-20. Those are 4 mm apart in frame PD, which is 2 mm of decentration per lens. The lens width you see in the product title is the less important half of the pair.

What these numbers do not tell you

They govern horizontal fit, which is where most online orders go wrong, but they are silent on several things that also matter.

They say nothing about lens height, which decides whether the frame covers your field of view and whether progressive lenses will fit at all — a progressive generally needs at least 28 to 30 mm of vertical lens depth. They say nothing about how the bridge is shaped, which matters enormously for whether a frame sits level on your particular nose, and which is why two frames with identical bridge widths can feel completely different. They say nothing about pantoscopic tilt, wrap, or the fitting height that a progressive lens is ordered to.

This tool performs arithmetic on measurements you supply. It is not a dispensing service and is not optometric advice. For progressive or bifocal lenses, strong prescriptions, prescriptions containing prism, or children's eyewear, have the frame fitted by a qualified dispensing optician.

Questions people ask

Where do I find the numbers on my glasses?

Inside one of the temple arms, the part that goes over your ear. Take the glasses off, turn an arm so the inner face points up, and look near the hinge. You will see something like 52-18-140. On some frames the numbers are printed inside the bridge instead, or on the back of the nose pad area.

What do 52, 18 and 140 mean?

Lens width, bridge width and temple length, all in millimetres. Lens width is the widest horizontal measurement of one lens opening. Bridge width is the gap between the two lenses. Temple length is the full length of the arm, including the bend that goes over the ear.

What is a frame's PD, and why does it matter more than lens width?

Lens width plus bridge width gives the distance between the geometric centres of the two lens openings, which is effectively the frame's own pupillary distance. Comparing that to your PD tells you how far each lens must be shifted so its optical centre sits in front of your pupil. That is the number that governs whether a frame works for you.

Is a few millimetres of decentration a problem?

Usually not. Optical labs decentre lenses as routine work, and up to about three millimetres per lens is invisible in practice. Past roughly five millimetres it starts to show, and how much it shows depends on lens power: below about two dioptres it is largely academic, while above four it drives both edge thickness and comfort.

Can I use the same numbers to buy frames online?

Yes, and it is the most reliable way to shop. Take the numbers from a pair you already know fits, add the first two together, and look for frames whose lens width plus bridge width lands within about three millimetres of that total. Most retailers let you filter by lens width and bridge.

Does this replace a proper fitting?

No. These numbers govern horizontal fit, which is the part most often got wrong when buying online. They say nothing about frame depth, how the bridge sits on your nose, pantoscopic tilt, or the fitting height needed for progressive lenses. For progressives or a strong prescription, see a dispensing optician.