Tools / Corneal curvature
Corneal Curvature Converter
Convert corneal radius to diopters and back with a clinical reference table for common values.
Last updated: March 2026
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
- User enters radius in mm (e.g. 7.80)
- Tool computes: 337.5 ÷ 7.80 = 43.269…
- Displays rounded result: 43.27 D
From power to radius
- User enters power in D (e.g. 43.27)
- Tool computes: 337.5 ÷ 43.27 ≈ 7.799…
- 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.