How MM and Diopter Values Describe Corneal Curvature
Why corneal curvature is expressed in both mm and diopters
Corneal curvature is commonly expressed as a radius in millimeters or as an equivalent power in diopters. These are two ways to describe the same surface.
The relationship is inverse: a shorter radius means a steeper cornea and a higher dioptric value. A longer radius means a flatter cornea and a lower dioptric value.
This matters when you are comparing keratometry readings, topography outputs, and contact lens base curves. One device may report a meridian as 45.00 D, while another reports the same curvature as 7.50 mm. Converting between mm and diopters keeps interpretation consistent across instruments and ordering systems.
MM to diopter conversion formula
Most clinical keratometers use a standardized refractive index of 1.3375 (the keratometric index) to approximate corneal power from the anterior surface radius. With this convention, the conversion formulas are:
- MM to diopters: D = 337.5 ÷ r (with r in millimeters)
- Diopters to mm: r = 337.5 ÷ D
Example: 7.50 mm corresponds to 45.00 D, and 43.00 D corresponds to approximately 7.85 mm.
This tool uses the 1.3375 convention because it aligns with the SimK style values that are widely used when K readings and base curves are communicated for contact lens fitting and ordering.
MM to diopter conversion chart
This reference table shows common keratometry values converted between millimeters and diopters using the standard 1.3375 keratometric index (D = 337.5 ÷ r). Use it as a quick chairside lookup when comparing K readings across instruments or reconciling base curve labels with topography values.
| Radius (mm) | Power (D) | Description |
|---|---|---|
| 7.00 | 48.21 | Very steep |
| 7.10 | 47.54 | Very steep |
| 7.20 | 46.88 | Steep |
| 7.30 | 46.23 | Steep |
| 7.40 | 45.61 | Moderately steep |
| 7.50 | 45.00 | Moderately steep |
| 7.60 | 44.41 | Average steep |
| 7.70 | 43.83 | Average |
| 7.80 | 43.27 | Average |
| 7.90 | 42.72 | Average |
| 8.00 | 42.19 | Average flat |
| 8.10 | 41.67 | Moderately flat |
| 8.20 | 41.16 | Moderately flat |
| 8.30 | 40.66 | Flat |
| 8.40 | 40.18 | Flat |
| 8.50 | 39.71 | Very flat |
| 8.60 | 39.24 | Very flat |
Values use D = 337.5 ÷ r and are rounded to two decimal places. For exact values or radii not listed, use the calculator above.
How this supports base curve selection and RGP fitting
In soft lens fitting, base curve selection is usually not a strict one-to-one match to keratometry. Instead, you choose an initial base curve within the manufacturer's options and then confirm movement, centration, and comfort on eye. Converting between mm and diopters helps you compare your instrument readings to how trial sets and base curves are labeled.
In RGP fitting, the mm to diopter relationship is foundational for tear lens thinking and base curve adjustments. Converting between millimeters and diopters makes it easier to reconcile K readings, chosen base curve, and the optical implications of steepening or flattening. If you are designing ordered power around a selected fit, pair this with the RGP Calculator.
For converting a full spectacle prescription to a contact lens starting power (including vertex compensation), use the Glasses to Contact Lens Calculator.
Important note about instrument differences
Some topographers and tomographers report additional metrics that do not use the keratometric index, such as true net power or total corneal power. These can differ slightly from SimK even when the physical radius is the same. For contact lens ordering and most chairside comparisons, SimK-style values are typically the most compatible with how base curves are labeled.