To convert K readings to curvature in mm use this formula:
$$r = \frac{337.5}{K}$$
Where:
r = radius of curvature (mm)
K = keratometry reading (diopters)
337.5 = derived from the standard keratometric refractive index (n = 1.3375), where 337.5 = (1.3375 − 1) × 1000
| Average K (D) | Radius of Curvature (mm) | Suggested SilSoft BC |
| ≥ 45.0 D | ≤ 7.50 mm | 7.5 mm (steepest) |
| 43.0–44.9 D | 7.50–7.85 mm | 7.7 mm |
| ≤ 42.9 D | ≥ 7.86 mm | 7.9 mm (flattest) |
Age-Based Corneal Curvature
Newborns (0–2 weeks): Mean K ~49 D (r ≈ 6.9 mm), rapidly flattening to ~46 D by 8 weeks and ~44 D by 12 weeks
0–2 years: Pooled mean corneal radius is 7.41 mm (~45.5 D), which is steeper than older children
2–4 years: Mean K ~43.7 D (r ≈ 7.72 mm), approaching adult values
By age 3: Corneal curvature generally reaches the adult range (~43–44 D)