We carried out a retrospective study to evaluate the relationship between vitreous cavity length, graft size, and final spherical equivalent refraction after penetrating keratoplasty (PK) for keratoconus. We found a straight-line relationship between vitreous cavity length and spherical equivalent for a one-surgeon series using the same technique throughout. The use of 0.25-mm undersized grafts shifted the results an average of 2.2 Dioptres in a more hypermetropic direction (p = 0.07 for the whole group, p < 0.01 for paired eyes). Hence the final spherical equivalent following PK for keratoconus can be predicted. Also, by altering the size of the donor graft button, the final refraction can be manipulated to some extent towards acceptable ametropia or to match the refraction of the fellow eye.