Lens Thickness Estimator

This calculator estimates the edge thickness of a simplified plano-spherical lens blank. It is useful for exploring how optical power, refractive index, clear diameter, and center thickness interact in one idealized geometry.

It is not a finished-lens quotation, prescription check, or manufacturing calculation. A dispensing optician or lens laboratory needs the complete prescription, frame measurements, decentration, lens form, minimum-thickness rules, prism, and material data.

Method used

The model treats one surface as plano and the other as spherical. It first estimates the powered surface radius:

R = 1,000 × (n − 1) ÷ |P|

where R is radius in millimetres, n is refractive index, and P is power in dioptres. The factor 1,000 converts metres to millimetres.

It then calculates spherical sag using the semi-diameter, not the full diameter:

s = R − √(R² − (D ÷ 2)²)

For a minus lens, estimated edge thickness is CT + s. For a plus lens, it is CT − s. A zero-power input returns the entered center thickness. The calculator rejects impossible geometry where the radius is not larger than the semi-diameter or a plus-lens estimate would produce a non-positive edge.

Worked example

Use these inputs:

• power: −5.00 D;
• refractive index: 1.60;
• diameter: 65 mm;
• center thickness: 2.00 mm.

The radius is 1,000 × 0.60 ÷ 5 = 120 mm. The semi-diameter is 65 ÷ 2 = 32.5 mm. The spherical sag is:

120 − √(120² − 32.5²) = 4.4848... mm

Because this is a minus lens, add the sag to the center thickness:

2.00 + 4.4848... = 6.4848... mm

SoupCalc displays an estimated edge thickness of 6.48 mm.

What the result means

The result is the edge thickness of this one-surface model at the entered circular diameter. Within that model, a larger diameter or stronger absolute power increases sag. At the same power and diameter, a higher refractive index increases the calculated radius and generally reduces sag.

The diameter should represent the usable circular diameter being estimated. A frame's horizontal box measurement is not automatically the finished lens diameter, especially when decentration and frame shape are involved.

Accuracy and limitations

Real spectacle lenses usually have two curved surfaces rather than one plano surface. Their thickness can differ materially because of base curve, back vertex power, aspheric or atoric design, cylinder and axis, prism, optical-center placement, effective diameter, bevel position, frame shape, safety minimums, and laboratory rounding. Refractive index alone also does not determine weight, impact resistance, chromatic aberration, coatings, or suitability.

Do not use this estimate to order, edge, certify, or modify eyewear. Do not infer that the thinnest numerical result is the safest or optically best choice. Ask the dispensing professional or laboratory for an actual thickness calculation based on the selected frame and complete prescription.

Sources

• Edmund Optics — SAG calculator and spherical sag equation
• ZEISS — Eyeglass lens materials and refractive index
• ZEISS — Factors that affect finished lens thickness

Editorial record

Written and manually reviewed by the SoupCalc Editorial Team. The formula and worked example were checked against the current calculator implementation. Last reviewed: August 10, 2026. No external eye-care reviewer participated. This page remains unavailable for indexing and advertising until all approved localizations pass review.