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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.
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.
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
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
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
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.
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).
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).
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.
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.
The difference between the two K readings quantifies corneal astigmatism. This helps determine if toric contact lenses or astigmatism-correcting spectacle lenses are needed.
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.
In ophthalmology settings, K readings are essential for calculating intraocular lens power before cataract surgery. Accurate Ks lead to better refractive outcomes after surgery.
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.
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.
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.
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.
Often combined with keratometry in modern kerato-refractometers.
How K readings translate into contact lens base curve decisions.
Deep dive into astigmatism classification and clinical significance.
Browse all CPO and CPOA study topics organized by category.
Keratometry (K) readings measure the curvature of the anterior (front) surface of the cornea. They are expressed in diopters of refractive power and/or in millimeters of radius of curvature. Since the cornea is responsible for about two-thirds of the eye's total refractive power, its curvature significantly affects overall vision. K readings are taken in two meridians (the steepest and flattest) to characterize both the overall curvature and any corneal astigmatism.
Normal K readings for the average adult cornea range from approximately 42.00 to 45.00 diopters, corresponding to a radius of curvature of about 7.5 to 8.0 millimeters. Values steeper than 47-48 D may suggest keratoconus or other corneal ectasia and warrant referral. Very flat readings (below 40 D) are less common and may indicate previous refractive surgery or unusual corneal anatomy.
K readings are the starting point for selecting the base curve of a contact lens. For soft lenses, the base curve is typically chosen slightly flatter than the flattest K reading. For rigid gas permeable (RGP) lenses, the fitting relationship between the lens base curve and the corneal curvature is more critical -- the lens may be fit "on K" (matching the flat K), slightly steeper, or slightly flatter depending on the fitting philosophy. K readings also reveal how much corneal astigmatism is present, which influences whether a toric lens design is needed.
With-the-rule (WTR) astigmatism means the steeper corneal meridian is near 90 degrees (vertical) -- the cornea is shaped more like a football lying on its side. Against-the-rule (ATR) astigmatism means the steeper meridian is near 180 degrees (horizontal). WTR astigmatism is more common in younger patients, while ATR becomes more prevalent with age. Oblique astigmatism has the steep meridian between 30-60 or 120-150 degrees. These distinctions matter for contact lens selection and spectacle correction.
The conversion formula is D = 337.5 / r, where D is the dioptric power and r is the radius of curvature in millimeters. For example, a cornea with a 7.5 mm radius has a power of 337.5 / 7.5 = 45.00 D. Conversely, a cornea measuring 43.00 D has a radius of 337.5 / 43.00 = 7.85 mm. The constant 337.5 is derived from the assumed refractive index of the cornea (1.3375). This conversion is commonly tested on paraoptometric exams.