Keratometry measures the curvature of the central front surface of the cornea. The instrument treats the cornea like a convex mirror: it reflects illuminated targets (mires) off the tear film and measures their size to work out curvature. Readings are taken in the two principal meridians — the steepest and the flattest.
K readings are a starting point for contact lens base-curve selection, they're used with biometry for intraocular lens calculations, and they help track changes in the corneal surface over time.
Two units: millimeters and diopters
Keratometers report curvature as a radius (mm), as power (D), or both. Convert with:
D = 337.5 ÷ r(mm) · r(mm) = 337.5 ÷ D
The constant comes from the standard keratometric index of 1.3375. The relationship is inverse: a smaller radius is a steeper cornea with more diopters.
| Radius (mm) | Power (D) |
|---|---|
| 7.00 | 48.21 |
| 7.30 | 46.23 |
| 7.50 | 45.00 |
| 7.67 | 44.00 |
| 7.80 | 43.27 |
| 8.00 | 42.19 |
| 8.20 | 41.16 |
Recording K readings
Record the power of each principal meridian with its meridian, for example 43.00 @ 180 / 44.50 @ 090.
Note the quality of the mires too: clear, regular mires suggest a smooth surface, while distorted or irregular mires
are worth documenting and bringing to the practitioner's attention.
Corneal astigmatism
The amount of corneal astigmatism is the difference between the two meridians. The type depends on which meridian is steeper:
- With-the-rule (WTR): the vertical meridian (near 090) is steeper.
- Against-the-rule (ATR): the horizontal meridian (near 180) is steeper.
- Oblique: the steeper meridian falls between those (roughly 30–60° or 120–150°).
Examples
| Readings | Amount | Type |
|---|---|---|
43.00 @ 180 / 44.50 @ 090 | 1.50 D | with-the-rule |
42.00 @ 090 / 43.50 @ 180 | 1.50 D | against-the-rule |
Technique reminders
- Focus the eyepiece for your own eye before measuring.
- Make sure the patient is comfortably positioned and fixating the target.
- A blink before measuring helps smooth the tear film.
- Check calibration periodically, following the manufacturer's instructions.
Frequently asked questions
What does a keratometer measure?
A keratometer measures the curvature of the central front surface of the cornea in its two principal meridians, reported as a radius in millimeters or as power in diopters.
How do you convert keratometry from millimeters to diopters?
Divide 337.5 by the radius in millimeters. For example, 337.5 ÷ 7.50 mm = 45.00 D. The 337.5 comes from the standard keratometric index of 1.3375.
What is with-the-rule astigmatism?
Corneal astigmatism is with-the-rule when the vertical meridian (around 090) is steeper than the horizontal meridian. Against-the-rule means the horizontal meridian (around 180) is steeper.
Is a higher K reading steeper or flatter?
Steeper. More diopters means a smaller radius of curvature and a more steeply curved cornea.
Preparing for the contact lens exam? See the NCLE exam FAQ. For COA candidates: how to study for the COA exam.