What Is Corneal Power Estimation Keratometry?

How Corneal Power Is Estimated

Keratometry treats the cornea like a convex mirror and measures reflected target size to determine anterior curvature. The measured radius is converted to diopters using a keratometric index, commonly 1.3375, which yields the standard constant used in many conversions. Mean keratometry is often calculated from the steep and flat meridians to summarize central corneal power. This method is designed for consistency, not perfect physical measurement of both corneal surfaces.

Mean K and Astigmatism Terms

Keratometry reports two principal meridians and their orientation.

• Flat K, often labeled K1, with an axis in degrees
• Steep K, often labeled K2, with an axis in degrees
• Mean K, which summarizes overall central curvature

The difference between K2 and K1 estimates corneal astigmatism magnitude, and the axes describe the meridian orientation.

Where Corneal Power Estimation Is Used

Estimated K values support several common clinical decisions. They are used to choose a starting base curve for contact lenses and to describe corneal astigmatism trends over time. Biometry devices often incorporate keratometry as an input for intraocular lens (IOL) power calculations for cataract surgery. Clinicians may also compare K values with corneal topography or corneal tomography when irregularity is suspected.

When the Estimate Can Be Wrong

Errors are more likely when the relationship between anterior and posterior corneal curvature is abnormal. After laser vision correction, anterior-only keratometry can misestimate total corneal power and affect IOL planning unless post-refractive methods are used. Keratoconus and other irregular corneas can distort the reflected targets and create unstable readings. In these settings, tomography-based total corneal power metrics and repeat measurements after ocular surface optimization are often preferred.