Every pair of quality prescription glasses or sunglasses carries three numbers stamped along the inner temple arm, usually separated by a small square: something like 48 □ 20 - 145.
These aren’t serial numbers or batch codes. They represent the foundational dimensions of the frame in millimeters—specifically the lens width, bridge distance, and temple arm length.
However, two frames with the exact same “48-20-145” stamp can fit completely differently on your face. A chunky acetate frame and an ultralight wire frame might share identical lens numbers, but one will feel roomy while the other squeezes your temples.
Here is a breakdown of what each dimension actually measures, why total frame width is often the deciding factor, and how to choose the right proportions for your face or brand collection.
1. The Three Numbers on the Temple Arm
The stamped numbers follow an international optical standard (the ISO Boxing system). They always appear in the same order:
| Inscription | Dimension | What It Measures | In 48 □ 20 - 145 |
|---|---|---|---|
| 48 | Lens Width | Horizontal width of each lens opening | 48 mm |
| □ | ISO Standard | Hallmark certifying standard boxing measurement | Standard |
| 20 | Bridge Width | Shortest distance between the two lenses | 20 mm |
| 145 | Temple Length | Total arm length from hinge screw to tip | 145 mm |
The small square symbol (□) simply verifies that the dimensions were taken using the standard bounding-box method rather than older, non-standard workshop techniques.
Lens Width (48 mm)
Lens width is the horizontal measurement across the widest part of one lens opening.
Beyond aesthetics, this number directly affects your prescription. A smaller lens width (44 mm – 48 mm) keeps high-prescription lenses thinner and lighter at the outer edges, making it the preferred range for round vintage frames and strong corrective wear. A 49 mm to 52 mm lens width serves as the standard unisex baseline for optical retail, while sunglasses typically expand to 54 mm – 58 mm or larger to provide adequate UV coverage.
Bridge Width (20 mm)
Bridge width is the shortest distance between the two inner lens edges, directly over the bridge of your nose.
This is the most critical parameter for comfort because the bridge bears the weight of the glasses. Unlike temple arms, which can often be adjusted with heat, an acetate bridge is rigid.
- 15 mm – 17 mm: Suited for narrow facial profiles or higher, slender nasal bridges.
- 18 mm – 20 mm: The standard baseline fitting the majority of adult wearers.
- 21 mm – 23 mm: Best for broader nasal roots or bold, architectural acetate designs.
Temple Length (145 mm)
Temple length measures the full extended length of the arm, starting from the center of the front hinge barrel and following the curve down to the tip behind the ear.
Standard unisex adult frames typically use 140 mm or 145 mm arms. Petite women’s styles often use 135 mm, while extended frames designed for broader head profiles require 150 mm to 155 mm to sit comfortably behind the ear without pulling the front tight against the temples.
2. Why Two “48-20” Frames Can Fit Differently: Total Frame Width
A common question in frame selection is why an acetate 48-20 frame fits comfortably, while a metal 48-20 frame in the same shape feels tight.
The reason is Total Frame Width—the full distance from outer hinge to outer hinge across the front. The stamped numbers only measure the lens and bridge; they don’t account for the thickness of the frame rim and endpieces.
- Thin Titanium Frame (48-20):
(48 mm × 2) + 20 mm bridge + 4 mm of thin hinge lugs = ~124 mm total width - Chunky 8mm Acetate Frame (48-20):
(48 mm × 2) + 20 mm bridge + 14 mm of extended acetate endpieces = ~138 mm total width
That 14 mm difference completely changes how the frame feels on the head. When evaluating a frame, always look at total width alongside boxed lens dimensions:
| Total Frame Width | Fit Category | Typical Facial Profile |
|---|---|---|
| 125 mm – 132 mm | Narrow / Petite | Slender faces, petite women, teenagers |
| 133 mm – 139 mm | Medium / Standard | Universal adult unisex fit |
| 140 mm – 146 mm | Wide | Broader jawlines, larger men’s styles |
| 147 mm+ | Extra Wide / Oversized | Broad facial structures, statement sunglasses |
3. How to Measure Your Frames Accurately
If you already own a pair of glasses that fits you well, you can measure it with a simple millimeter ruler to find your target numbers:
- Lens Width: Measure horizontally across one lens at its widest point, staying inside the frame groove.
- Bridge Width: Measure the narrowest gap between the two lenses, directly above the nose pads.
- Total Frame Width: Lay the glasses face down on a flat surface and measure the straight-line distance between the outer left hinge and outer right hinge.
- Temple Arm Length: Measure along the arm from the hinge screw to the bend, then follow the curve down to the tip.
4. Standard Fit vs. Low Bridge Fit (Asian Fit)
Most fit complaints don’t come from head width; they happen at the bridge. If a wearer has a flatter nasal crest or higher cheekbones, standard European-fit frames often slip down the nose or rest on the cheeks, lifting the glasses every time the wearer smiles.
To solve this, frame manufacturers engineer Low Bridge Fit (commonly marketed as Asian Fit or Universal Fit):
| Design Element | Standard Fit | Low Bridge Fit (Asian Fit) |
|---|---|---|
| Nose Pad Projection | Standard 2.5 mm – 3.0 mm | Built-up 5.0 mm – 6.5 mm pads |
| Bridge Contact Area | Narrower, steeper nasal seat | Broader, flatter contact surface |
| Metal Pad Arms | Straight short stems | S-curved gooseneck adjustable arms |
| Pantoscopic Tilt | Standard 8° – 10° forward tilt | Flatter 4° – 6° tilt (clears cheekbones) |
Built-up pads elevate the frame front slightly higher and further away from the face. This creates breathing room above the cheeks and keeps eyelashes from brushing against the back of the lenses.
5. Size Planning for Eyewear Collections
For brands developing a commercial lineup, balancing your size spread across key styles helps ensure wide demographic appeal:
| Category | Lens Width | Bridge | Temple | Total Width | Target Demographics |
|---|---|---|---|---|---|
| Petite Optical | 45 – 47 mm | 18 – 20 mm | 135 – 140 mm | 128 – 132 mm | Slender profiles, strong prescriptions |
| Unisex Standard | 48 – 51 mm | 19 – 21 mm | 140 – 145 mm | 135 – 139 mm | Core retail volume, fits most adults |
| Executive / Men’s | 52 – 55 mm | 20 – 22 mm | 145 – 150 mm | 140 – 145 mm | Broader profiles, classic aviators/rectangles |
| Sun / Statement | 54 – 60 mm | 18 – 22 mm | 145 – 150 mm | 144 – 152 mm | Fashion sunglasses, wrap silhouettes |
6. Specifying Dimensions in Custom Manufacturing
When working with an eyewear manufacturer on custom styles, you don’t need complete 3D CAD models to get started. A straightforward design brief containing:
- Reference imagery or hand sketches showing the silhouette and front profile.
- Target dimensions (e.g., 49-20-145 with a target total width of 138 mm).
- Bridge preference (standard molded pads, built-up Asian fit pads, or adjustable metal arms).
- Target material (cellulose acetate, titanium, stainless steel, or TR90).
From those parameters, factory optical engineers can draft a precise 2D technical drawing and produce 3D-printed prototypes for real-world fit testing before opening hard production tooling.
Technical Consultation & Frame Sizing Support
Getting frame proportions right early in development prevents costly tooling adjustments later. If you are developing a new optical or sunglasses collection and want feedback on sizing specifications, the Hermitin engineering team can review your tech packs and advise on material tolerances and bridge ergonomics.
- Manufacturing overview: Custom Eyewear OEM/ODM Process
- Direct inquiry: Reach out to our team on WhatsApp or follow our WhatsApp Channel for production updates and material releases.