Diopters ↔ radius conversion
K readings and base curves, either direction, any index.
About this calculator
The keratometer measures the radius of the front corneal surface and reports it as a power. It assumes a refractive index of 1.3375 instead of the cornea’s true 1.376, and the lower value allows for the minus power of the back surface against the aqueous.
Contact lens base curves are written in millimeters of radius, so a K reading in diopters has to be converted before it can be compared with a lens. The same relation runs either way and works with any index.
Formula
r (mm) = (n − 1) × 1000 / K, which is 337.5 / K at n = 1.3375
Worked example
A K reading of 45.00 D is 337.5 / 45.00 = 7.50 mm.
Sources
- Geometric, Physical and Visual Optics, ch. 14, Reflection (keratometry)
- Clinical Procedures for Ocular Examination, ch. 6, Contact Lenses (keratometry)