This document discusses sample size calculations for case-control studies. It provides an example comparing the rate of diabetes mellitus (DM) between patients with cataracts (cases) and those with normal vision (controls). Based on literature finding a 50% DM rate in cases and 8% in controls, the required sample size is 17 cases and 17 controls to detect this difference with 80% power and 5% type 1 error rate. Manual calculations and online calculators can both be used to determine sample size for case-control studies. Prior information is needed on exposure rates in cases and controls to perform these calculations.