A strategy has been developed for optimizing the design of runners in injection molds with multiple identical cavities. This approach is based on mathematically rigorous derivations using the power-law viscosity model under conditions where the pressure drop in runners is not significantly influenced by temperature variation, The design rule to minimize runner volume for specified injection pressure and rate is simple: The diameter of any runner downstream from a junction is determined by dividing the diameter of the upstream runner by the cube root of the number of downstream branches at the junction. This rule has been extended to noncircular runners using the hydraulic radius approximation and the requirement of equal velocities in the circular and noncircular runners, Minimum volume cross-sectional shapes have been determined for U-shaped and trapezoidal shaped runners. For circular runners a method for estimating the pressure drop for non-isothermal flow quantifies the deviation from the pressure drop for isothermal flow at the melt temperature. As runner size decreases, the trend is for viscous heating to dominate over cooling and for the estimated nonisothermal pressure drop to be laver than the isothermal pressure drop.