Eyeglass Frame Size Numbers Explained (and How They Help You Pick a Case)
Those small numbers printed inside your glasses, such as 52-18-140, are a surprisingly useful tool. They describe the frame size, and they can help you choose replacement frames and the right case.
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Look at the inside of one of your glasses temples, the arms that go over your ears. You will usually find a short line of numbers, often printed alongside the brand, model and color code. A typical example looks like 52□18 140, or 52-18-140. These numbers are measurements in millimeters and they tell you a lot about the frame.
The three main numbers
Lens width (first number)
The first number is the horizontal width of each lens, measured at its widest point. Common values range from around 40 mm for small frames to 60 mm or more for oversized frames and sunglasses.
Bridge width (second number)
The second number is the distance between the two lenses, across the bridge that sits on your nose. Typical values range from around 14 to 24 mm. The square symbol, when present, is a convention indicating the boxing system used to measure the lens.
Temple length (third number)
The third number is the length of the temple arm, from the hinge to the end of the arm, including the curve behind the ear. Common lengths are 135, 140, 145 and 150 mm.
Other markings you may see
| Marking | Meaning |
|---|---|
| Model name or number | Identifies the frame style |
| Color code | Identifies the frame color variant |
| Lens height (sometimes) | Vertical height of the lens |
| CE mark | Indicates conformity marking used in the European market |
| "Made in" | Country of manufacture |
Lens height is not always printed, but it is often listed by retailers. It matters for progressive lenses, which need enough height for the different viewing zones.
Estimating total frame width
Total frame width, measured across the front from the outside edge of one end piece to the other, is the number that matters most for fit and for choosing a case. You can estimate it as:
Total width ≈ (2 × lens width) + bridge width + a few millimeters for the frame edges and end pieces
For a 52-18-140 frame:
- 2 × 52 = 104 mm
- 104 + 18 = 122 mm
- Plus around 6 to 14 mm for the rims and end pieces = roughly 128 to 136 mm
This is an estimate. Thick acetate frames and frames with prominent end pieces will be wider. Measuring with a ruler is always more accurate.
Size ranges at a glance
| Frame size | Typical lens width | Typical total width | Case implication |
|---|---|---|---|
| Small | Under 50 mm | Under 128 mm | Fits most standard and slim cases |
| Medium | 50 to 54 mm | About 128 to 138 mm | Standard medium case |
| Large | 55 to 58 mm | About 138 to 146 mm | Large case; check length |
| Oversized | 59 mm and up | Over 146 mm | Large or oversized case |
These ranges are approximate and vary by frame design.
From frame numbers to case size
For choosing a case, you need three folded dimensions: width, height and depth. The temple numbers give you a starting estimate for width.
- Estimate total frame width using the formula above.
- Add clearance. For a hard case, look for outside length about 10 to 15 mm larger than the folded width.
- Check height. Use the lens height if listed, plus the frame rim.
- Measure depth. Frame numbers do not tell you depth, which depends on frame curvature and temple design. Measure it directly, especially for wraparound frames.
Our full case sizing guide explains clearance in detail, and the guide to oversized and wraparound frames covers large shapes.
Using the numbers to buy new frames
If your current glasses fit well, their numbers are a helpful reference when shopping for new frames:
- Lens width within a few millimeters of your current pair usually gives a similar overall size.
- Bridge width affects how the frame sits on your nose. A difference of a couple of millimeters can change comfort.
- Temple length affects how the arms rest behind your ears. Opticians can sometimes adjust temple fit.
Your optician or eye care provider can help with fitting, especially for progressive lenses.
What the numbers do not tell you
- Frame depth and curvature, which matter for case fit.
- Frame thickness, which affects folded height and depth.
- Weight.
- How the frame fits your face shape and nose.
Checklist
- Found the three numbers on the inside of a temple.
- Noted lens width, bridge width and temple length.
- Estimated total frame width.
- Measured folded height and depth directly.
- Added sensible clearance when choosing a case.
Worked examples
Here is how the formula plays out for a few common frame sizes. These are estimates for illustration; actual frames vary with rim thickness and end piece design.
| Frame marking | Lens + lens + bridge | Estimated total width | Likely case size |
|---|---|---|---|
| 48-20-140 | 48 + 48 + 20 = 116 mm | About 122 to 130 mm | Standard or slim case |
| 52-18-140 | 52 + 52 + 18 = 122 mm | About 128 to 136 mm | Standard medium case |
| 55-17-145 | 55 + 55 + 17 = 127 mm | About 133 to 141 mm | Medium to large case |
| 58-16-145 | 58 + 58 + 16 = 132 mm | About 138 to 146 mm | Large case |
| 62-15-135 (sunglasses) | 62 + 62 + 15 = 139 mm | About 145 to 153 mm | Large or sunglasses case |
Remember that sunglasses and sport frames often add significant depth because of their curve, so the folded depth may be the deciding factor rather than width.
How to measure frames with no markings
If the markings have worn off, measure the glasses directly:
- Lens width: Measure one lens horizontally at its widest point, inside the rim.
- Bridge width: Measure the shortest distance between the two lenses.
- Temple length: Measure along the temple from the hinge screw to the tip, following the curve.
- Folded width, height and depth: Fold the glasses and measure as described in our case sizing guide.
A flexible tailor's tape makes the temple measurement easier, but a ruler works if you measure the straight section and the curved section separately and add them.
Children's frame sizes
Children's frames follow the same numbering, with smaller values. Because children grow quickly, frames may be replaced often; recheck case fit when a new pair arrives, especially with flexible or wraparound kids' frames.
Frequently asked questions
What does 52-18-140 mean on glasses?
It means a lens width of 52 mm, a bridge width of 18 mm and a temple length of 140 mm.
Where do I find my glasses size?
Usually on the inside of one temple arm. Some frames print it on the bridge or the inside of the frame front.
What if my glasses have no numbers?
Measure directly with a ruler in millimeters. Measure one lens horizontally, the gap between lenses, and the length of a temple arm.
Can I choose a case using only these numbers?
They give a good estimate of width, but measure folded height and depth directly for the best fit.
Bottom line
The numbers on your glasses describe lens width, bridge width and temple length in millimeters. Use them to estimate total frame width, then measure depth directly to choose a case that fits.
If you own several pairs, compare their numbers side by side. The pair that fits most comfortably is a useful reference for future purchases, and its case is a good starting point for sizing new cases.