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Omnikalc

Keratometry calculator (corneal power)

Enter the radii of curvature of the flattest and steepest corneal meridians, the axis of the flat meridian and the refractive index to get the power of each meridian, the mean keratometry, the corneal astigmatism and the axis of the steep meridian.

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Results

Corneal astigmatism

1.41 D

Moderate astigmatism, usually corrected with glasses or a toric lens
Power of the flat meridian
42.99 D
Power of the steep meridian
44.41 D
Mean keratometry
43.70 D
Axis of the steep meridian
90 °
Total corneal power with the Gullstrand index (1.376) at the mean radius
48.67 D

The keratometry calculator converts the radii of curvature of the cornea into dioptric powers, and gives the mean keratometry, the corneal astigmatism and the steep axis.

How the calculation works

  • Enter the flat and steep radii in millimeters.
  • Enter the axis of the flat meridian and the index.
  • Read the powers, the astigmatism and the axis.

Example

Radii of 7.85 mm and 7.60 mm with an index of 1.3375 give (1.3375 − 1) / 0.00785 = 42.99 D and 44.41 D. The mean keratometry is 43.70 D and the corneal astigmatism is 1.41 D. With the flat meridian at 180°, the steep meridian is at 90°, which is with-the-rule astigmatism.

Why it matters

The calculator helps optometry and ophthalmology students, contact lens fitters, cataract surgery teams and anyone who reads a keratometry report and wants to understand the numbers.

Good to know

Keratometry measures only the central few millimeters of the front surface, so it misses irregular corneas such as keratoconus, scars or previous refractive surgery, where topography or tomography is needed. The mean value is used in intraocular lens power formulas, and an error of 1 diopter in the keratometry causes about 0.7 diopter of error in the postoperative refraction. Corneal astigmatism may differ from the total refractive astigmatism because of the lens and the back surface of the cornea. For a toric lens the posterior corneal astigmatism, which tends to be against-the-rule, should be considered. Always use the instrument values, the same unit and the same index, when comparing measurements. Measurements should be repeated on both eyes, and the values are compared with the previous visit and with the refraction of the patient.

The spherical equivalent calculator converts prescriptions, the vertex distance calculator converts lens power, the IOP correction calculator uses corneal thickness and the visual acuity calculator converts notations.

Frequently asked questions

How is corneal power computed?

The keratometer measures the radius of curvature of the front surface and converts it to a power with a fictitious index: K = (n − 1) / r, with n = 1.3375 and r in meters. A radius of 7.80 mm gives 43.27 D.

Why is the index 1.3375?

It is a compromise that accounts for the weak negative power of the back surface of the cornea, so that the front surface radius gives the approximate total corneal power. The true index of the cornea is 1.376, and the back surface is usually assumed or measured separately.

Which axis is which?

The steep meridian is perpendicular to the flat one. With-the-rule astigmatism has the steep meridian near 90°, against-the-rule near 180°. The result gives the steep axis from the axis of the flat meridian.

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Results are estimates provided for information purposes. They do not replace professional advice.

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