An algorithm for estimation of the posterior corneal curvature is presented and applied on data from normal and keratoconic eyes. Radius of central posterior corneal curvature are demonstrated to be (mean +/- SD) 6.71 +/- 0.23 mm and 5.58 +/- 0.78 mm in normals and keratoconic eyes, respectively. This corresponds to a ratio between posterior and anterior corneal curvature at 0.85 and 0.83 in the groups mentioned. Both these ratios are significantly smaller than the corresponding ratio at 0.88 in Gullstrand's schematic eye which on corneal dioptric power results in offset errors at 0.20 D and 0.46 D in normal and keratoconic eyes. It is further demonstrated that the ratio is not constant over the corneal surface, resulting in central peripheral dioptric offset errors between 0.2 D and -0.31 D in normals and between 0.46 D and -0.38 D in keratoconic eyes. On corneal dioptric power it is finally shown that a variation in the refractive index of aqueous humor has a 30 times larger influence than a similar variation in corneal refractive index.