Glasses Frame Size Calculator
Three numbers off the frame, one off your prescription. Nothing you type leaves the browser.
A frame's size is stamped on the inside of one temple arm as three numbers — lens width, bridge width and temple length, written like 52□18-140. This tool takes the first two, plus the pupillary distance from your prescription, and runs the published check on them.
It does the arithmetic most guides leave to you: the frame's own pupillary distance, the decentration each lens needs, the verdict against the two published ceilings, and the smallest lens blank the front can be cut from. Every intermediate value is printed, so you can check each step with your own numbers.
The stamp and your number
The first stamped number — the 52 in 52□18-140.
The second stamped number — the 18 in 52□18-140.
On your prescription, or from a shop that measured you before.
The vertical height of the lens opening, in millimetres. Not stamped — a shop can measure it.
Fill lens width, bridge and your pupillary distance to get the reading.
Calculated in your browser — the numbers you type are never sent to a server.
What the tool runs, step by step
Frame PD = lens width + bridge width. The two stamped numbers give the frame its own pupillary distance — a 52□18 front is 70 mm (2.76 in) before your face enters the picture.
Decentration per eye = (frame PD − your PD) ÷ 2. That same 70 mm front on a 64 mm (2.52 in) pupillary distance moves each lens 3 mm (0.12 in) inward; on a 63 mm (2.48 in) one, 3.5 mm (0.14 in).
The verdict reads the per-eye figure against the two published ceilings — under 5 mm per eye where possible, and the tighter 3 to 4 mm band. In frame terms: a gap of 10 mm or less satisfies the 5 mm ceiling, and a gap of 6 mm or less satisfies both.
The lens blank starts at the effective diameter, plus the difference between the frame PD and yours, plus 2 mm. The effective diameter is never smaller than the lens width; its rectangular upper bound is the square root of (lens width² + depth²), which runs slightly larger than a real rounded lens. Stocked diameters are 65, 70 and 75 mm — past 75 mm, the failure mode is a lens that cannot be cut.
Where these rules come from
Every rule above is published on this site, spelled out in one of these guides — nothing here is this page's own invention, except the rectangular upper bound, which is labelled as one wherever it appears:
- What the numbers on glasses mean — the subtraction, the two published ceilings and the blank rule this tool mirrors
- How to measure glasses frame size — reading a frame with a tape, and the widest lens a bridge can pay for
- Bridge width glasses — where the 14–24 mm band behind the second input is published, and the two rules that guess it without a tool
- Why are my glasses lenses so thick — what the blank decision costs once a minus prescription is in play
- Glasses temple length — the third stamped number, and why it enters none of the sums above