Spectacle Lens Thickness Calculator

Work out spectacle lens thickness instantly with clear inputs, formula shown and shareable results.

About Spectacle Lens Thickness Calculator

Uses the sagitta approximation to estimate how thick a lens becomes at the edge for a minus prescription, or at the centre for a plus prescription.

Formula

Sagitta approximation

Sag (mm) = semi-diameter^2 (mm) x |power| / (2000 x (n - 1)); minus lens edge = centre thickness + sag

Worked example

With the default values below, Spectacle Lens Thickness Calculator returns Edge thickness of 4.57. Change any input above and every figure here is recalculated in your browser.

Inputs used

Lens poweraccepts -20–20 D: -4 D
Lens diameteraccepts 30–80 mm: 60 mm
Refractive indexaccepts 1.4–1.9: 1.586
Centre thickness for a minus lensaccepts 0.6–5 mm: 1.5 mm

Results

Edge thickness: 4.57 mm
Sagittal depth: 3.07 mm
Difference from the centre: 3.07 mm
Thickest point: 4.57 mm

Step by step

1Values used
2Sagitta approximation
3Edge thickness= 4.57 mm
4Sagittal depth= 3.07 mm
5Difference from the centre= 3.07 mm
6Thickest point= 4.57 mm

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Educational estimate only. Not a substitute for professional clinical judgement, and must not be used to make treatment or dosing decisions.
This is an estimate for general information and not professional medical advice. See the full disclaimer for the limits that apply to this kind of calculator.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Educational estimate only. Not a substitute for professional clinical judgement, and must not be used to make treatment or dosing decisions.

Frequently Asked Questions

How much does a higher index help?

Going from 1.50 to 1.74 reduces the sag by roughly 40% for the same power and diameter.

Does frame choice matter?

Yes. A smaller lens diameter reduces thickness far more effectively than index alone.