The cup-to-disc ratio (often notated CDR) is a measurement used in ophthalmology and optometry to assess the progression of glaucoma.

The optic disc is the anatomical location of the eye's "blind spot", the area where the optic nerve leave and blood vessels enter the retina.

But glaucoma, which is in most cases associated with an increase in intraocular pressure, often produces additional pathological cupping of the optic disc.

As glaucoma advances, the cup enlarges until it occupies most of the disc area.

Deep but stable cupping can occur due to hereditary factors without glaucoma.