About the Sunglasses Size Calculator
Sunglasses and eyeglass frames are sized using a standard three-number system stamped inside one temple (arm), usually written as three numbers separated by dashes — for example 52-18-140. Reading left to right, those are the lens width (also called eye size), the bridge width (the gap between the lenses that sits over your nose), and the temple length (the arm, from hinge to tip), all measured in millimeters. This calculator uses those manufacturer numbers together with your own face width to estimate how a given frame will actually fit.
The formula
There is no single lens on a frame that spans your whole face, so retailers and opticians estimate a frame's overall front width with a simple rule of thumb:
- Total frame width ≈ (2 × lens width) + bridge width + ~4 mm. The extra 4 mm accounts for the hinges and the thin plastic or metal rim around each lens, split roughly 2 mm per side. For a 52-18-140 frame: (2 × 52) + 18 + 4 = 126 mm.
- Fit gap = total frame width − face width. A gap between about −5 mm and +5 mm is a comfortable match. A more negative gap means the frame will likely pinch at the temples; a more positive gap means it may sit loosely or slide down the nose.
Face width is measured horizontally across your face at its widest point — typically temple to temple or across the cheekbones — with a flexible tape or a ruler held level in a mirror. Averaging two or three measurements reduces error, since a single reading can be off by a few millimeters.
Standard size categories
Optical retailers commonly group adult face widths into three bands, each with a typical range of lens width, bridge width, and temple length. These are industry conventions rather than a physical law, so treat them as a starting shortlist, not a guarantee:
- Narrow — face width under 129 mm: lens width 46–48 mm, bridge 14–18 mm, temple 135–140 mm.
- Medium — face width 129–139 mm: lens width 48–52 mm, bridge 18–20 mm, temple 140–145 mm.
- Wide — face width over 139 mm: lens width 52–56 mm, bridge 20–24 mm, temple 145–150 mm.
Practical context
These formulas estimate frame front width and a starting size category from the numbers printed on a frame or in a product listing — they cannot account for lens height, temple curvature (whether the arm bends down behind the ear), face shape, or how a specific style sits on your nose bridge. Use the result to shortlist candidates, then confirm fit with an in-person try-on, a store's virtual try-on tool, or a generous return policy before committing to a purchase.