What is the normal size of an optic disc in diameter?

The horizontal diameter of an optic disc is 1.8mm. Optic disc is normally slightly veically oval with an average area of approximately 2.7mm2. At the optic disc, all the retinal nerve fibers merge into the optic nerve and the central retinal vessels enter and leave the eye here. At the optic disc there is complete absence of photoreceptors. Optic cup: It is a slightly eccentric cavitation of the optic nerve that has a slightly flattened oval shape corresponding to that of the neuroretinal rim. It is the brightest pa of the optic disc. Size of the optic cup correlates with the size of the optic disc, larger the optic disc larger the optic cup. Ref: Ophthalmology: A Pocket Textbook Atlas By Gerhard K. Lang, M.D., 2nd edn, page 374-5