Future electric and hybrid electric vertical takeoff and landing vehicles will require high-efficiency and lightweight electric motor-driven propulsion systems. One key to designing an optimum electric motor-driven propulsion system, is to have a method for estimating the mass and efficiency of a gearbox for different power levels, motor speeds, and propeller speeds. Understanding the trade space for gearbox mass and efficiency allows the overall optimization of the propulsion system. A number of past papers have detailed gearing theory, sizing, and efficiency calculations (Ref. 1). Past work at NASA has explored the optimum sizing of gearsets (Ref. 2). More recently, genetic optimization tools have been used to optimize gearboxes for terrestrial traction applications (Refs. 3–6). Genetic optimization algorithms are ideal for gearbox design due to the number of optimization variables that create discontinuities in the design space (Ref. 5). In this paper, a preliminary analytical gearbox optimization tool for electric vertical takeoff and landing vehicles is presented. Example gearbox sizing studies are carried out with the tool for electric multirotor type vehicles based on the NASA Revolutionary Vertical Lift Technology (RVLT) project 6-passenger quadrotor vehicle (Ref. 7). The section “Gearbox Genetic Optimization” discusses the details of the optimization tool. Example tool results are presented in the section “Example Design Optimization Results” for multirotor-type vehicles based on the RVLT quadrotor concept vehicle. The “Conclusion” section outlines future tool improvement and development plans.