Precise IOL power calculation is essential for optimal benefits of implant surgery. Prior to1975, IOL power was calculated on the basis of clinical history, i.e. pre-operative refractive error prior to development of cataract. This lead to errors in over 50% of cases. However, a number of formulae are now available to accurately calculate the IOL power required in a patient. All these formulae are based on an accurate measurement of the corneal power and the axial length.