Analytical predictions of primary implant power using presumptive errors in keratometer and axial length measurements were performed using the modified Binkhorst, modified Colenbrander, Holladay, Hoffer, and SRK II equations. These predictions demonstrate that the contributions to primary implant power error resulting from inaccurate axial length and keratometer measurements are algebraically additive. In eyes with a normal axial length, the resulting implant power determination error can be larger than differences in implant power prediction among these five IOL equations. Calculations using measurement errors of 0.2 mm in axial length and 0.50 diopter (D) in corneal curvature predicted a worst case primary implant power error of +/- 1.17 D. These calculations were performed using an axial length and corneal curvature near the population mean. In contrast, implant equation variability was determined to be +/- 0.19 D by calculating the standard deviation of the five implant power formulas with the measurement errors set to zero. Implant power prediction errors were increased when a flat cornea was paired with an axial hyperopic or an axial myopic eye. These combinations maximize the implant power error resulting from both implant formula variation and inaccurate measurements. Primary implant power error prediction tables are presented for emmetropic, axial hyperopic, and axial myopic eyes, as a function of presumed errors in axial length and corneal curvature. These error predictions clearly show that inaccuracy in axial length measurements and keratometer readings can be first order determinants of postoperative spherical refractive error.