Introduction: A good postoperative outcome of cataract surgery requires a precise biometric measurement; hence, accurate preoperative measurements of the eye are essential to achieving the desired postoperative refractive result. This study assessed the effect of pupillary dilatation on ocular biometry and intraocular lens (IOL) power calculation in a Nigerian Teaching Hospital. Method: A prospective study was performed on eighty eyes of 69 patients scheduled for cataract surgery. Anterior chamber depth (ACD), axial length (AL), flat keratometry (K1), steep K (K2), average keratometry reading (average K), Lens thickness (LT), Intraocular lens (IOL) power were measured, before and after pupil dilatation using Apamide plus (Tropicamide 1% Phenylephrine 2.5%. Intraocular lens (IOL) power was calculated using the Sanders-Retzlaff-Kraff/Theoretical (SRK/T) and Haigis formulae integrated in the ultrasound Tomey Biometer AL-100. We compared the ocular parameters before and after pupil dilatation. Results: There was a statistically significant increase in mean post-dilatation ACD and a decrease in mean post-dilatation LT in all patients, in comparison to mean pre-dilatation measurements (p. < 0.05). IOL power calculation using SRK/T and Haigis formula was not affected by pupillary dilatation. However, in all 3 groups of AL, only group 2 (AL) had a statistically significant increase in mean post-dilatation in ACD, in comparison to mean pre-dilatation measurements (p. < 0.05). Conclusions: Pupillary dilatation significantly affected biometry readings like ACD and LT but did not influence keratometry readings, AL and IOL Calculation formulae SRK/T and Haigis.