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What Is a Keratometry Conversion Table?

A keratometry conversion table is a chart that converts corneal curvature from radius in millimeters to keratometric power in diopters, and sometimes back again. Most tables use the standard keratometric index convention of 1.3375, which produces the 337.5 constant in common formulas. Clinicians use these tables to quickly interpret keratometry readings for contact lens fitting, corneal assessment, and surgical planning. The conversion is a standardized estimate based mainly on the anterior corneal surface, so it may not match total corneal power in every eye.

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What Is a Keratometry Conversion Table?

A keratometry conversion table is a chart that converts corneal curvature from radius in millimeters to keratometric power in diopters, and sometimes back again. Most tables use the standard keratometric index convention of 1.3375, which produces the 337.5 constant in common formulas. Clinicians use these tables to quickly interpret keratometry readings for contact lens fitting, corneal assessment, and surgical planning. The conversion is a standardized estimate based mainly on the anterior corneal surface, so it may not match total corneal power in every eye.

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What the Table Converts

The table links two ways of describing the same corneal curvature. A smaller radius means a steeper cornea and a higher diopter value, while a larger radius means a flatter cornea and a lower diopter value. Many tables also help you compare the steep and flat meridians that describe corneal astigmatism.

  • Radius of curvature in millimeters
  • Keratometric power in diopters, often shown as K readings
  • Two principal meridians for astigmatism comparisons

How the Conversion Is Calculated

Most conversion tables are built from the standard relationship between radius and keratometric power using a keratometric index. With the 1.3375 convention, power is calculated as D = 337.5 / r where r is in millimeters. To convert back, the table uses r = 337.5 / D. Small differences can occur if a device uses a different index or reports total corneal power instead of SimK values.

Common Examples

Tables often include quick reference points that are widely used in clinic. For example, a radius of 7.50 mm corresponds to about 45.00 D under the 1.3375 convention. A radius of 8.00 mm corresponds to about 42.19 D, which reflects a flatter cornea. These examples help clinicians sanity-check readings and communicate curvature consistently across reports.

Limitations and Special Cases

Conversion tables assume a typical relationship between the front and back of the cornea, so they can be less accurate when that relationship is altered. This commonly happens after laser vision correction, corneal transplantation, or in irregular corneal conditions. In those cases, tomography-based measures that include posterior curvature may be more reliable than simple conversions. Your clinician will choose the metric that best matches the decision being made.

FAQs on Keratometry Conversion Tables

Why is the constant 337.5 used?

The constant comes from the standard keratometric index convention of 1.3375 used to estimate corneal power from the anterior radius. It simplifies the calculation so clinicians can convert between millimeters and diopters quickly. The value is a convention, not a direct measurement of true total corneal power.

Do all keratometers use the same keratometric index?

Many instruments use 1.3375, but some devices use slightly different indices or report additional values such as total corneal power. Because of this, the same physical cornea can show small differences across reports. When precision matters, use the values and settings from the specific device that measured the eye.

Can I use a conversion table after LASIK or PRK?

It can be misleading because standard keratometric assumptions may not match true corneal power after refractive surgery. Post-refractive eyes often need specialized methods and tomography-based measurements for surgical planning. Your clinician will decide which approach is appropriate for your situation.

What is the simplest way to convert diopters back to millimeters?

Use the inverse relationship r = 337.5 / D, where r is in millimeters and D is in diopters. Conversion tables list these pairs so you can cross-check without doing the math. This is mainly intended for SimK style keratometry values.

References

Keratometer. StatPearls (National Library of Medicine, National Institutes of Health). https://www.ncbi.nlm.nih.gov/books/NBK580516/. Date Accessed February 4 2026.

Keratometry. Cleveland Clinic. https://my.clevelandclinic.org/health/diagnostics/24267-keratometry. Date Accessed February 4 2026.

Diopter to Radius Conversion Chart. Acculens. https://acculens.com/wp-content/uploads/2021/12/diopter-radius-chart.pdf. Date Accessed February 4 2026.

Diopters to Radius Conversion Chart. IGEL. https://www.igel.com.my/wp-content/uploads/2021/07/Diopters-To-Radius-Conversion-Chart_c.pdf. Date Accessed February 4 2026.

Diopters to Radius Conversion. EyeKnowOD. https://eyeknowod.com/wp-content/uploads/2023/03/Diopters-to-Radius.pdf. Date Accessed February 4 2026.