Keratometry is the measurement of the curvature of the front surface of the cornea. Since the cornea provides roughly two-thirds of the eye's total refractive power, knowing its exact shape is essential for contact lens fitting, astigmatism assessment, surgical planning, and monitoring corneal health. In most optometry practices, the paraoptometric performs keratometry as part of the standard pre-testing workup.
K readings appear on the CPO and CPOA exams in several contexts: understanding what the numbers mean, knowing normal ranges, converting between diopters and millimeters, and understanding how K readings guide clinical decisions. If your office uses a kerato-refractometer (which combines autorefraction with keratometry), you may be obtaining K readings every day without giving them much thought. The exam will make sure you actually understand what those numbers represent.
This article covers the principles of keratometry, manual and automated measurement techniques, the all-important conversion formula, normal values, clinical applications, and the types of astigmatism that K readings reveal.
What Keratometry Measures
A keratometer measures the curvature of the anterior corneal surface by reflecting light off the cornea and analyzing the size and shape of the reflected image. The cornea acts like a convex mirror -- when a known-size target (called a mire) is reflected off the cornea, the size of the reflected image tells you how curved the cornea is. A steeper (more curved) cornea produces a smaller reflected image; a flatter cornea produces a larger one.
The measurement is taken in two principal meridians -- the steepest and the flattest. In a perfectly spherical cornea, both meridians would have the same curvature. In a cornea with astigmatism, the two meridians differ, and the amount of that difference is the corneal astigmatism.
Understanding K Reading Values
Diopters (D)
K readings in diopters represent the refractive power of the cornea. Normal range is approximately 42.00 to 45.00 D. Higher values mean a steeper (more powerful) cornea; lower values mean a flatter one.
Average: ~43.50 D
Steep (>47 D): Possible keratoconus
Flat (<40 D): Possible prior surgery
Millimeters (mm)
K readings in millimeters represent the radius of curvature. Normal range is approximately 7.5 to 8.0 mm. Smaller radius means a steeper curve; larger radius means flatter. Contact lens base curves are specified in mm.
Average: ~7.75 mm
Steep (<7.0 mm): Possible keratoconus
Flat (>8.5 mm): Less common
The Conversion Formula
D = 337.5 / r (mm), or equivalently, r = 337.5 / D. The constant 337.5 comes from the assumed corneal refractive index of 1.3375. This formula is frequently tested on the CPO and CPOA exams.
Example 1: Cornea radius = 7.50 mm. Power = 337.5 / 7.50 = 45.00 D
Example 2: Cornea power = 43.00 D. Radius = 337.5 / 43.00 = 7.85 mm
Example 3: Cornea radius = 8.00 mm. Power = 337.5 / 8.00 = 42.19 D
Manual Keratometer Operation
The manual keratometer (such as the classic Bausch & Lomb or Marco models) uses a system of mires -- illuminated targets that reflect off the cornea. The operator looks through the eyepiece and adjusts dials to align the reflected mire images.
Step 1: Focus and Position
Focus the eyepiece for your eye. Position the patient at the chin rest with forehead against the bar. Adjust the instrument height so the mires are centered on the cornea. The reflected mire images should appear as plus signs or circles (depending on the instrument model).
Step 2: Align the Mires
Rotate the instrument barrel until the plus signs or mires are aligned horizontally -- this sets you to one of the principal meridians. Turn one measuring dial to overlap or align the mire images in the horizontal meridian (this reads the horizontal curvature). Then turn the other measuring dial to align the vertical mire images (this reads the vertical curvature).
Step 3: Record the Readings
Read the diopter values from both dials and note the axis of each meridian. Record as two K values with their axes, for example: 44.00 @ 180 / 45.50 @ 090. The difference between the two readings is the amount of corneal astigmatism (in this case, 1.50 D). The axis of the flatter meridian is typically noted as the axis of the astigmatism.
Clinical Applications of K Readings
Contact Lens Base Curve
K readings are the starting point for selecting the base curve of a contact lens. The relationship between the lens curve and the cornea determines how the lens fits -- too steep and it binds; too flat and it slides.
Astigmatism Assessment
The difference between the two K readings quantifies corneal astigmatism. This helps determine if toric contact lenses or astigmatism-correcting spectacle lenses are needed.
Corneal Irregularity Screening
Distorted or irregular mires on a manual keratometer suggest corneal surface irregularity. Very steep readings may indicate keratoconus. These findings warrant referral for corneal topography.
IOL Calculation
In ophthalmology settings, K readings are essential for calculating intraocular lens power before cataract surgery. Accurate Ks lead to better refractive outcomes after surgery.
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Types of Corneal Astigmatism from K Readings
With-the-Rule (WTR) Astigmatism
The steep meridian is at or near 90 degrees (vertical). This is the most common pattern in younger patients. Think of the cornea as shaped like a football lying on its side -- steeper vertically than horizontally. The minus cylinder axis in the spectacle Rx is near 180 degrees. Example K readings: 43.00 @ 180 / 44.50 @ 090.
Against-the-Rule (ATR) Astigmatism
The steep meridian is at or near 180 degrees (horizontal). More common in older patients as the cornea tends to flatten vertically with age. The cornea is steeper horizontally -- like a football standing on end. The minus cylinder axis is near 90 degrees. Example K readings: 44.25 @ 180 / 43.00 @ 090.
Oblique Astigmatism
The steep meridian falls between 30-60 degrees or 120-150 degrees. Less common than WTR or ATR. Oblique astigmatism can be more visually disturbing and harder to correct with contact lenses because the meridians are not aligned with the natural lid position that helps stabilize toric lenses. Example: 43.50 @ 045 / 45.00 @ 135.
When K Readings Suggest Referral
While interpretation is the doctor's responsibility, paraoptometrics should be aware of K reading patterns that may indicate pathology. Very steep readings (above 47-48 D), especially if accompanied by high or irregular astigmatism, may suggest keratoconus or other corneal ectasias. Significant asymmetry between eyes (one eye much steeper than the other) is also a red flag.
On a manual keratometer, distorted mires that cannot be properly aligned suggest irregular corneal astigmatism. This is an important qualitative finding -- even if you cannot quantify the irregularity, noting that the mires appeared distorted helps the doctor determine if corneal topography or further evaluation is needed.