BACKGROUND Although relation between Lens Opacities Classification System III (LOCS III) and nuclear density is established, no data are available about nuclear size at different LOCS III grades. OBJECTIVE The aim of this study is to evaluate the relation between LOCS III grading of nuclear opacity obtained preoperatively and the size of the nucleus obtained during cataract surgery. METHODS This was a prospective observational study carried out in a hospital attached to medical college. METHODS Patients who underwent manual small-incision cataract surgery or extra-large temporal tunnel cataract extraction and gave consent were included in this study. Institutional Ethics Committee clearance was taken for the study. Preoperative LOCS III grading was obtained at slit-lamp biomicroscope. Ocular dimensions were obtained by preoperative immersion biometry. The thickness and diameter of the nucleus obtained by extraction were measured up to 10 μ accuracy. Data were analyzed for the change in nuclear thickness, nuclear diameter, age, lens thickness, and anterior chamber depth in relation to the LOCS III grade of the nucleus. METHODS Statistical analysis used in this study was one-way ANOVA, mean, and range. RESULTS There was a significant increase (P < 0.05) in nuclear thickness, nuclear diameter, and age with increasing LOCS III grade of the nucleus. The change in nuclear size was linear between Grades 1 and 4. The nuclear size did not increase between Grades 4 and 5. It increased steeply from Grade 5 to Grade 6.9. CONCLUSIONS LOCS III grading of the nucleus can be utilized for determining the nuclear thickness and diameter preoperatively. These data can be helpful in adjusting machine parameters during phacoemulsification.