OH

Clinical reference

Optometry Hub

Tools / Corneal curvature

Corneal Curvature Converter

Convert corneal radius to diopters and back with a clinical reference table for common values.

Corneal curvature tool

Radius and diopter conversion

Convert corneal radius and diopters with a quick clinical reference.

Radius to diopters

Power

43.27 D

Diopters to radius

Radius

7.80 mm

Understanding corneal curvature conversions

Corneal curvature can be expressed as radius (mm) or refractive power (D). A steeper cornea has a shorter radius and higher dioptric power, while a flatter cornea has a longer radius and lower power.

Conversion formulas

Power (D) = 337.5 ÷ radius (mm)

Radius (mm) = 337.5 ÷ power (D)

How the Cornea Curvature Converter works

This tool is a bidirectional corneal curvature converter that instantly translates between two ways of describing the front surface of the cornea:

Radius of curvature (mm)

Physical measurement from keratometry or topography

Refractive power (D)

Optical power the cornea contributes to the eye's refraction

It uses the universally accepted keratometric formula and includes a clinical reference table for quick lookup of common values.

Why this conversion matters in optometry

Corneal curvature is the strongest refracting surface of the eye, contributing ~43 D on average (about 70% of total refractive power).

  • • Keratometry (K-readings) are usually reported in both mm and D.
  • • Contact lens fitting (especially rigid gas-permeable lenses) relies on matching the lens base curve to the cornea's radius.
  • • LASIK, orthokeratology, and keratoconus monitoring all use these values daily.

The tool removes manual calculation errors and lets you switch units instantly.

Inputs & outputs

Radius to Diopters

Input: Corneal radius (mm) → Output: Power (D)

Example: 7.80 mm → 43.27 D

Diopters to Radius

Input: Power (D) → Output: Radius (mm)

Example: 43.27 D → 7.80 mm

A live clinical reference table displays common values (e.g. 7.50 mm = 45.00 D, 8.00 mm = 42.19 D).

Mathematical formulas

Power (D) = 337.5 ÷ radius (mm)

Radius (mm) = 337.5 ÷ Power (D)

Where does 337.5 come from?

Derived from the keratometric refractive index n = 1.3375. The formula P = (n – 1) / r becomes 337.5 / r when working in mm.

Step-by-step calculation

From radius to power

  1. User enters radius in mm (e.g. 7.80)
  2. Tool computes: 337.5 ÷ 7.80 = 43.269…
  3. Displays rounded result: 43.27 D

From power to radius

  1. User enters power in D (e.g. 43.27)
  2. Tool computes: 337.5 ÷ 43.27 ≈ 7.799…
  3. Displays: 7.80 mm

The tool automatically updates the opposite field in real time (bidirectional).

Clinical applications

  • • Soft contact lens fitting – convert K-readings to expected tear lens power.
  • • RGP / Ortho-K lens design – choose base curve (BC) usually 0.05–0.15 mm flatter than flat K.
  • • Calculating corneal astigmatism – difference between steep K and flat K (in D or mm).
  • • Pre-LASIK screening – compare measured K to expected values.
  • • Keratoconus monitoring – track steepening (smaller radius = higher D).

Important notes

  • • This is anterior corneal power only. True total corneal power (used in IOL calculations) requires Scheimpflug or OCT data and a different index (~1.376 for posterior surface).
  • • The constant 337.5 is standard but some older instruments or regions use 336.0 or 338.0 – always check your keratometer manual.
  • • Measurements assume a spherical surface. Real corneas are toric; use flat K and steep K separately for astigmatism.

Limitations & clinical disclaimers

  • • For contact lens base curve selection, clinicians usually add 0.5–1.0 D flatter than flat K (or convert back to mm). This tool gives the raw conversion – final lens choice always needs trial fitting.
  • • Not a diagnostic tool – always correlate with topography, pachymetry, and refraction.