The three size numbers on glasses usually describe lens width, bridge width and arm length, in millimetres. A marking such as 52 □ 18 140 means a 52 mm lens width, an 18 mm bridge and 140 mm arms. These are frame measurements, not your prescription.
Knowing what they mean makes comparing glasses online more straightforward. Start with the markings on a pair you already wear, then compare them with the measurements and design of the frames you are considering.
A quick guide to 52 □ 18 140
52: lens width
The first number gives the horizontal width of one lens. It does not include the surrounding rim or describe the total width of the glasses.
18: bridge width
The second number gives the distance between the lenses at the bridge. This is one part of the design that influences how a frame sits around your nose.
140: arm length
The third number gives the length of the side arm, also called the temple, from the hinge to the tip, including its bend.
The small square is a separator. Some frames use a dash, and the third measurement may appear elsewhere on the frame.
Where should you look?
Check the inside of the arms first. You may also find markings around the bridge. Good lighting can make small or faded printing easier to read. A close-up photograph can help if the lettering is difficult to see.
Write down the complete marking before shopping. Keep the brand and model reference too, so you can distinguish the frame you own from a different design with similar measurements.
Which numbers are the model and colour?
A frame may carry several codes. A brand or model reference identifies the design, while another code can identify its colour. The measurements are a separate piece of information.
For example, a product listing might offer sizes 53 and 55. These commonly refer to lens width. Check the product’s measurement details to confirm, rather than assuming that either number is the complete frame width.
How to use your current pair as a reference
- Choose a useful starting pair. If it fits comfortably, its measurements give you a reference for comparing alternatives.
- Record all the available measurements. Look beyond the first number and include bridge width and arm length.
- Compare the design as well as the numbers. A deep square lens and a shallow rectangular lens can look quite different even with the same lens width.
- Check what is missing. If a listing does not include the information you need, ask before ordering.
If you dislike the fit of your current pair, do not use its size as a target without considering what needs to change. An optician can help distinguish a frame that needs adjustment from one whose size or design is unsuitable.
Why identical numbers do not guarantee identical fit
The size markings describe selected dimensions. They do not capture every detail of a frame, such as the thickness of its rims, the shape of its bridge or the way the arms curve.
Think of them as a way to narrow your shortlist. Check photographs, available dimensions and fit information together.
Adding two lens widths and the bridge measurement does not reliably give the full outside width of the frame, because the rims and end pieces also contribute.
Are these numbers the same as pupillary distance?
No. Frame dimensions describe the glasses. Pupillary distance describes the distance between your pupils and is a separate measurement.
Do not substitute a frame number for a measurement requested when ordering prescription lenses.
Before you choose your next pair
Keep a note of your current model, colour and size together. When you find a frame you like, compare those details with its product information and check the lens options separately.
Explore glasses at Urban Optics and use the measurements to make a more informed shortlist.
If you are also considering a different shape, our guide to choosing glasses for your face shape offers another starting point.
