Abstract Purpose An accurate estimation of normal bladder capacity can be helpful in evaluating the patient with genitourinary disease and in interpreting urodynamic data. Prior studies have provided initial estimates. We propose 2 new equations that are practical, easy to use and more accurate than those previously published. Materials and Methods We retrospectively reviewed the records of more than 5,000 children undergoing radionuclide cystography at our institution. Radionuclide cystography was conducted by instilling 99m technetium pertechnetate via gravity drip in awake children. Bladder capacity was believed to be achieved when rate of inflow diminished to a minimal rate, initiation of voiding occurred or significant discomfort was indicated. Patients with vesicoureteral reflux, infravesical obstruction, urinary tract infection, dysfunctional voiding or other lower urinary tract pathology were excluded from the study. Linear and nonlinear regression modeling established the relationship between age and bladder capacity. Results A total of 2,066 children (598 boys and 1,468 girls) had normal radionuclide cystography and were included in the analysis. Analysis of variance demonstrated that increasing age was strongly predictive of bladder capacity (p <0.0001). Because a nonlinear model was the most accurate formula for all ages (4.5 × age 0.40 = capacity [ounces]), 2 practical linear equations were determined: 2 × age (years) + 2 = capacity (ounces) for children less than 2 years old, and age (years) [divided by] 2 + 6 = capacity (ounces) for those 2 years old or older. Although girls had larger capacities than boys, the rate of increase was not significantly different between them. Conclusions The relationship between normal bladder capacity and age in children follows a nonlinear curve. This nonlinear relationship can be approximated by 2 practical linear formulas that are easy to remember and are derived from a larger population than any prior study. These formulas provided accurate estimations of bladder capacity when prospectively applied to normal patients.