Scleral contacts are noticeably larger than standard gas permeable (GP) contacts and have a diameter equal to or greater than that of soft contact lenses. The smallest sclerals are approximately 14.5 mm in diameter, and the largest can be up to 24 mm.
Large-diameter scleral and semi-scleral GP lenses rest on the sclera and vault over the misshapen cornea of a person with keratoconus, for better vision.
Typically, lenses that are 18 mm or smaller are subcategorized as mini-sclerals.
The average human cornea is approximately 11.8 millimeters in diameter, so even the smallest scleral contacts are designed to cover the entire corneal surface.