A glasses size printed on the arm is a three-number sequence in millimeters — lens width, bridge width, and temple length — that tells you exactly how the frame is built to fit your face.
The wrong reading sends you back to the store. The right one saves an hour of trial and error. Every pair of prescription glasses and sunglasses uses the same sizing system, stamped on the inside of the temple arm. Here is how to decode whatever sequence is written on yours — and what to do when the numbers have worn off.
What Do The Numbers On Glasses Arms Mean?
The sequence — like 52–20–150 — is read from left to right, always in millimeters.
- First number (Lens Width / Eye Size): The horizontal width of one lens at its widest point. Standard range is 40–60 mm.
- Second number (Bridge Width): The distance between the two lenses where the frame rests on your nose. Typical range is 14–24 mm.
- Third number (Temple Length): The length of the arm from the hinge to the tip, including the bend behind the ear. Most common are 135, 140, 145, and 150 mm.
Some manufacturers — JINS, Eyebuydirect, and RX Safety among them — add a fourth number for Lens Height, the vertical measurement of the lens. This number matters most if you need bifocals or progressive lenses.
Where To Find The Size On Your Glasses
Flip your glasses over and look along the inside surface of either temple arm — the part that runs from the frame front to behind your ear. The numbers are etched, stamped, or printed on a thin strip of plastic or metal. They are usually small but legible without magnification. If the etching is faded or worn off, manual measurement with a millimeter ruler is the only fallback.
How To Measure When The Numbers Are Gone
Grab a standard millimeter ruler. These four measurements will match what was originally printed:
- Lens Width: Measure horizontally across one lens at its widest spot. Values between 40 and 62 mm are typical.
- Bridge Width: Measure the gap between the lenses at the closest point on the frame. Most are 14–24 mm.
- Temple Length: Measure from the hinge screw to the very tip of the arm, following the bend. Expect 120–150 mm.
- Lens Height: Measure vertically at the tallest point of the lens. If you wear progressives, this must be 30 mm or higher.
Glasses Size Ranges At A Glance
| Measurement | Standard Range | What That Means For You |
|---|---|---|
| Lens Width | 40–60 mm | Small ≤50 mm, Medium 51–54 mm, Large ≥55 mm |
| Bridge Width | 14–24 mm | Most common 18–20 mm; fit is flexible ±2 mm of your current frame |
| Temple Length | 120–150 mm | Common sizes: 135, 140, 145, 150 mm |
| Lens Height | Variable | Critical for progressives — must be ≥30 mm |
| Frame Width | 100–180 mm | Not printed on the arm; measure hinge-to-hinge manually |
How To Use Your Size To Pick The Right Frame
Size codes are a starting point, not the final word. The bridge measurement is the most forgiving — staying within 2 mm of your current frame works fine. The temple length should match within 2–3 mm. For the lens width, values within 2–3 mm of your current pair are safe.
The gap you need to check manually is the overall frame width, because it never appears in the printed code. Measure hinge to hinge across the full front of the frame. The frame width should not extend past your temples, and you should be able to slide no more than one finger between your face and the start of the temple arm. If the frame is too wide, glasses slide down your nose; too narrow and they pinch.
For readers who prefer a front-only design that skips the arm fit challenge entirely, our guide to the best armless reading glasses available now covers the top options that rest on the nose without temple pressure.
Common Mistakes When Reading Glasses Sizes
The most frequent error is mistaking the first number for the total frame width. That number is only the width of one lens — the full frame width is always wider and must be measured separately.
Other slip-ups include assuming the numbers are in centimeters (they are always millimeters), ignoring the fourth lens-height number when ordering progressives, and trusting faded etching instead of measuring manually.
Progressive Lens Frames: The Hard Requirement
If you wear progressive lenses, lens height is non-negotiable. The frame must provide at least 30 mm of vertical lens space for the gradient to work.
Safety Glasses and Non-Standard Frames
Laser safety glasses and industrial eyewear from brands like NoIR use their own sizing charts. Standard frame sizing still applies, but fit tolerances are tighter because optical alignment matters for protection. Always check the brand’s specific guide before assuming a standard reading fits.
Reading Fit: What Acceptable Tolerance Really Means
| Factor | Acceptable Variance | When It Causes Problems |
|---|---|---|
| Bridge width | ±2 mm | Outside 2 mm causes sliding or nose pinch |
| Lens width | ±2–3 mm | Affects peripheral coverage and style fit |
| Temple length | ±2–3 mm | Too short pulls the frame; too long loses grip |
| Frame width | Must not exceed temple width | Excess width causes slipping; under-width pinches |
A difference of a few millimeters may look minor but changes the contact pressure at your temples and nose bridge significantly.
FAQs
Are glasses sizes in centimeters or millimeters?
Every number printed on a glasses arm is in millimeters (mm). A reading of “52” means 52 mm, not 5.2 cm. This is consistent across all brands globally, including JINS, Warby Parker, and Zenni Optical.
What if my glasses have a fourth number on the arm?
A fourth number is the lens height, measured vertically at the tallest point of the lens. It is not always printed — JINS, Eyebuydirect, and RX Safety include it. For bifocals and progressive lenses, this number is critical and must be at least 30 mm.
How can I check my glasses size without looking at the arm?
Use a millimeter ruler to measure the lens width (horizontally across one lens), the bridge gap (between the lenses), and the temple arm length (hinge to tip). The overall frame width, which is never printed, requires a measurement hinge-to-hinge across the front.
Will my glasses fit if the numbers are a few millimeters off?
The bridge is the most flexible — staying within 2 mm of your current frame is safe. Lens width and temple length can vary up to 3 mm. Frame width is less forgiving: if the front extends past your temples, the glasses will slide.
References & Sources
- JINS Eyewear. “Guide to Eyeglass Frame Measurements.” Covers the full three-number system, measurement methods, and fit-tolerance values.
- Warby Parker. “Eyeglass Frame Sizes Explained.” Explains common fit mistakes and provides the 2–3 mm tolerance guideline.
- RX Safety. “How to Read Glasses Measurements.” Details the four-number system and the 30 mm progressive-lens requirement.
- Zenni Optical. “How to Measure Eyeglass Frame Size.” Step-by-step manual measurement instructions including frame width.