The challenge with post refractive corneas is that the Gullstrand ratio of the anterior to the posterior corneal curvature is no longer given. This leads to two main issues. Firstly, the K-readings are reported too flat with the cornea after an intervention to correct myopia, and too steep for a prior hyperopic patient. Secondly, in standard calculation formulae, the lens position is calculated based on AL and K-readings. Since the Ks no longer represent the initial cornea, the lens position calculation is wrong as well. Post-refractive formulae overcome both issues by correcting either the formula or the result for the issues mentioned.