Keratometry & Base Curve

Dioptre ↔ millimetre conversion, corneal astigmatism and sagittal depth for base curve selection.

Keratometry

Input unit

Flat K

42.50 D

7.94 mm

Steep K

44.00 D

7.67 mm

Mean K

43.25 D

7.80 mm

Corneal astigmatism

1.50 D

Minus-cyl axis ≈ 180° (steep at 90°)

Calculation

  1. 01Input: K1 42.5 D, K2 44 D
  2. 02Radii: 337.5 / 42.50 = 7.94 mm, 337.5 / 44.00 = 7.67 mm
  3. 03Corneal astigmatism = 44.00 − 42.50 = 1.50 D
  4. 04Mean K = (42.50 + 44.00) / 2 = 43.25 D

Sagitta / base curve geometry

Sagittal depth of a spherical surface over a given chord

Sagittal depth

3.747 mm

r = 8.60 mm · chord = 14.20 mm · surface power +39.24 D

s = 3.747 mmchord 14.20 mm
  1. 01y = chord / 2 = 7.1 mm
  2. 02s = r − √(r² − y²)
  3. 03s = 8.60 − √(73.96 − 50.41)
  4. 04s = 3.747 mm

Formulas

  1. 01D = 337.5 / r (mm)
  2. 02r (mm) = 337.5 / D
  3. 03Corneal astigmatism = Steep K − Flat K
  4. 04Mean K = (Flat K + Steep K) / 2
  5. 05Sagitta s = r − √(r² − (chord/2)²)

Clinical note

Parameter matching is not a fitting recommendation

The keratometric index 337.5 is a convention; some instruments use 1.3375 with slightly different results. Sagittal height comparison guides base curve choice but fit, movement, centration, comfort, corneal physiology and tear film must be assessed on-eye.

OptiX Pro provides mathematical calculations, reference information, and fitting assistance. It does not replace professional clinical examination, manufacturer fitting guidelines, or professional judgement. Contact lens selection and prescription must be clinically verified.