Lens Thickness Calculator: edge and centre thickness

Estimate how thick a spectacle lens will be at the edge and in the centre from its power, diameter and material, and compare materials from CR-39 to 1.74 high index. For opticians and optometrists.

The same lens in each material
Material	Index	Centre	Edge
CR-39 (1.50)	1.498	2 mm	9.7 mm
Crown glass (1.52)	1.523	2 mm	9.3 mm
Trivex (1.53)	1.53	2 mm	9.2 mm
Polycarbonate (1.59)	1.586	2 mm	8.5 mm
High index 1.60	1.60	2 mm	8.3 mm
High index 1.67	1.67	2 mm	7.6 mm
High index 1.74	1.74	2 mm	7.1 mm

How to use it

Enter the sphere and any cylinder (the ± button switches the sign), and the lens diameter in millimetres.
Pick the lens material, or enter another refractive index. Under Minimum thickness, set your lab's centre thickness for minus lenses and edge thickness for plus lenses.
Read the edge and centre thickness, the working, and the same lens in every material.

Formula

r = (n − 1) ÷ F (in metres; × 1000 for mm), sag = r − √(r² − y²) with y = half the diameter. The small-angle version, sag ≈ y² × F ÷ (2000 × (n − 1)) with y in mm, is shown alongside. Minus lens: edge = centre + sag. Plus lens: centre = edge + sag.

Worked example

A −6.00 D lens in CR-39 (n = 1.498), 70 mm across with a 2.0 mm centre: r = 0.498 ÷ 6 = 0.083 m = 83 mm, and the sag over y = 35 mm is 83 − √(83² − 35²) = 83 − 75.26 = 7.74 mm, so the edge is about 9.7 mm (the small-angle formula gives 7.38 mm). A +4.00 D CR-39 lens 60 mm across with a 1.0 mm edge needs a centre of about 4.7 mm.

Refractive indices used

CR-39 1.498, crown glass 1.523, Trivex 1.53, polycarbonate 1.586, and nominal 1.60, 1.67 and 1.74 for high-index plastics. Manufacturers' measured values differ slightly (for example 1.592, 1.661 and 1.732 for the MR-8, MR-10 and MR-174 monomers); use Other index for an exact figure.

References

Millodot M. Dictionary of Optometry and Vision Science, 8th ed. Elsevier, 2018: vertex depth (sagitta), s = r − √(r² − y²).
Pillay R, Hansraj R, Rampersad N. Historical development, applications and advances in materials used in spectacle lenses and contact lenses. Clin Optom 2020;12:157–167, Table 1 (refractive indices).

Formula checked on 7 October 2026. An estimate for eye-care professionals: confirm thickness with your lab.

Frequently asked questions

How is lens thickness worked out?

From the sag (sagitta) of the curved surface: s = r − √(r² − y²), where y is half the lens diameter and r = (n − 1) ÷ F is the radius that gives the power F in a material of index n. A minus lens is thinnest in the centre, so its edge is the centre thickness plus the sag; a plus lens is thinnest at the edge, so its centre is the edge thickness plus the sag.

How much thinner is high-index?

A higher index needs flatter curves for the same power, so the sag is smaller. For a −6.00 D lens 70 mm across with a 2.0 mm centre, the edge comes out at about 9.7 mm in CR-39 (1.50), 8.5 mm in polycarbonate (1.59), 7.6 mm in 1.67 and 7.1 mm in 1.74. The comparison table under the result works this out for your prescription.

What diameter should I enter?

For an uncut lens, its blank diameter. For a lens edged into a frame, what matters is the furthest edge from the optical centre: roughly the frame's effective diameter plus twice the decentration. A smaller frame or a well-centred lens gives a thinner edge.

Why does my lab quote a different thickness?

This is a thin-lens estimate for a round lens with all the power on one surface. Real lenses have a chosen base curve, aspheric designs, a frame shape that isn't round and the lab's own minimum thicknesses, all of which change the final figures a little.

What about astigmatism?

Enter the cylinder too: each principal meridian has its own sag, so the edge thickness varies around the lens. The calculator shows the thinnest and thickest edge.

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