Buying guide
Glasses frame sizes: what the three numbers mean
Most frames carry their size inside an arm: three numbers in millimetres, such as 52-18-145. This guide explains each one, how to turn them into the width across the front, and how to measure a pair you own when the print has worn away.
What do the numbers on my glasses mean?
Glasses carry their size as three numbers in millimetres, printed on the inside of one of the arms or on the inside of the bridge, often next to the model name and colour code. They look like 52□18-145, sometimes written 52-18-145, and they always come in the same order.
Lens width is the first number: the width of one lens at its widest point. It's also called the eye size. In 52-18-145 it is 52mm.
Bridge is the second: the gap between the lenses, where the frame sits on your nose. Here it is 18mm.
Temple length is the third: the length of the arm, from the hinge to the tip. Here it is 145mm.
The small square means the frame was measured with the boxing system, the international standard (ISO 8624) that draws a rectangle around each lens and measures that. Every listing on this site uses the same system, so the numbers compare directly.
Compare with a pair that fits
The easiest way to choose a size is to compare with glasses that already fit you. As a rough guide:
- Bridge is the number to take most seriously. Too wide and the frame slides down; too narrow and it sits high or pinches.
- Lens width within about 2mm of your current pair will usually feel similar. Larger reads wider on the face.
- Temple length is the most forgiving. A few millimetres either way rarely matters.
Lens width and bridge together set the width of the whole front, which is worth working out before you compare anything else.
Working out the total frame width
The width across the front does the most to decide whether a frame suits the width of your face, and it isn't printed. Work it out from the first two numbers: add the lens width twice and the bridge once. The temple length plays no part.
For 52-18 that is 52 + 52 + 18, about 122mm. The rim and hinges add a few millimetres at each side, so the frame itself measures a little more, but the sum is the figure to compare because it is worked out the same way on every frame. Three worked fronts:
- A small front, 46-20: 46 + 46 + 20, about 112mm. On this site a lens width of 46mm or under counts as narrow, and narrow frames gathers them in one place.
- A middle front, 52-18: about 122mm. Lens widths from 47mm to 52mm sit between the two; look for them among every frame we have online, where each listing gives its width across the front.
- A large front, 56-18: 56 + 56 + 18, about 130mm. From a 53mm lens width a frame counts as wide, and wide frames collects them.
Two things to keep in mind. There are two lenses, so a lens 2mm wider makes the front about 4mm wider. And lens width alone can mislead: a 50-22 and a 52-18 both come to about 122mm across the front, but the first puts more of that width into the bridge, so it sits differently on the nose. Compare the total first, then the bridge.
Every frame's listing gives its width across the front, worked out this way. Under Compare with a pair you own, type the three numbers from glasses that fit you and the listing shows, number by number and across the front, how many millimetres wider or narrower the frame is, or that it is the same.
If the numbers have worn off
The print inside an arm can wear away. A millimetre ruler and the pair you already wear will give you the same three numbers, near enough:
- Lens width: measure one lens straight across at its widest point. On a frame with a rim the lens sits in a groove, so the visible opening can measure a millimetre or so under the printed size.
- Bridge: measure the gap between the two lenses at their closest point.
- Temple length: measure one arm from the hinge to the tip, following the bend behind the ear. A soft tape measure makes this easier than a rigid ruler.
- Across the front: for a direct check, measure the front from one outer edge to the other. Expect a few millimetres more than lens width twice plus the bridge, for the rim and hinges.
Write the numbers down in the usual order, lens width, bridge, temple, and compare them with a listing as you would printed ones. An optician can also measure a frame for you, and you are welcome to ask us about the fit of a frame on this site.
Lens height and progressive lenses
Lens height is the depth of one lens, top to bottom. It isn't one of the three printed numbers and isn't marked on most frames, so we list it on each frame where we have it.
It matters most for progressive lenses, which fit distance, middle and reading zones into one lens, and for bifocals, which fit distance and reading. Many standard progressive designs ask for a lens height of about 28mm or more; shorter designs exist. Whether a particular frame is deep enough depends on the lens design and on your own measurements, so the optician fitting your lenses makes that call. Check with them before choosing a shallow frame.
Low bridge fit
Some frames are made for noses with a lower or flatter bridge, with a deeper bridge or longer nose pads so the frame sits off the cheeks and the lenses sit in front of the eyes rather than below them.
If frames tend to slide down your nose or rest on your cheeks, the bridge of a pair that sits well is the best starting point. You are welcome to ask us how a particular frame's bridge is made before you order.
Buying for a prescription? Read getting lenses for your frame before you order — a frame can't be returned once lenses have been fitted.
Questions we are asked
What does 52-18-145 mean?
It is the frame's size in millimetres, always in the same order. 52 is the lens width: one lens measured across at its widest point. 18 is the bridge: the gap between the two lenses, where the frame sits on your nose. 145 is the temple length: one arm, from the hinge to the tip. A small square between the first two numbers, as in 52□18-145, means the frame was measured with the boxing system in ISO 8624. Added together, 52 + 52 + 18 gives about 122mm across the front.
Which number matters most for fit?
For comfort on the nose, the bridge. Too wide and the frame slides down; too narrow and it sits high or pinches. For how the frame sits across your face, the width across the front, which you get by adding the lens width twice and the bridge once. Temple length is the most forgiving: a few millimetres either way rarely matters.
The quickest test is a pair that already fits. A lens width within about 2mm of yours will usually feel similar.
How do I work out total frame width?
Add the lens width twice and the bridge once. For 52-18-145 that is 52 + 52 + 18, about 122mm; for 46-20, about 112mm; for 56-18, about 130mm. The temple length plays no part. The frame itself runs a few millimetres wider, for the rim and hinges at each side, but the sum is the figure to compare, and every listing on this site gives it as the width across the front.
What lens height do progressive lenses need?
There is no single number. It depends on the lens design and on where your eyes sit in the frame, so the optician fitting your lenses decides, using your measurements. As a rough guide, many standard progressive designs ask for a lens height of about 28mm or more, and shorter designs exist for shallower frames.
Online we sell frames only, not lenses, and we can't promise that a frame will suit a particular prescription. We list lens height on each frame where we have it: check it with your optician before you buy.
What if the numbers have worn off?
Measure the pair you wear with a millimetre ruler. Lens width is one lens straight across at its widest point. Bridge is the gap between the lenses at their closest point. Temple length is one arm from the hinge to the tip, following the bend. Then compare them with a listing as you would printed numbers. An optician can also measure a frame for you, and you can ask us about the fit of a frame on this site.