Contact lenses are designed to fit on the front surface of your eye, called the cornea. The back curves of a contact lens, measured in millimetres, should match the curve of your eye.

  • Contact lenses are designed to fit on the front surface of your eye, called the cornea.
  • The back curves of a contact lens, measured in millimetres, should match the curve of your eye.
1

Significance

When you have your contact lens fitting exam, your eye doctor measures the curves of your eye with a keratometer. The measurements are called k-readings.

2

Fitting

Your eye doctor uses the k-readings to determine which contact lens's curve will give you the best vision, be the most comfortable, and keep your eye healthy.

3

Types

The base curve of a contact averages in the range of 8.0 to 9.5 millimetres, depending on the flatness or curves of the surface of your eye.

4

Features

Lenses are usually available in just a few base curves per style, such as 8.3, 8.6 or 8.9. The lower the number, the steeper of the curve, or angle, of the lens and your eye.

5

Difference between 8.6 and 8.9

A lens with a base curve of 8.6 is steeper than a lens with a base curve of 8.9. Many factors affect the size of a lens, including the prescription and the diameter (size across the lens), so a base curve in one type of lens may be slightly different in another type or brand of contact.