The range of hybrid electric aircraft in the series electric mode, i.e., with an engine generating power that charges a battery, which in turn powers the propellers of the aircraft, is deduced over the full spectrum of hybridization. For each degree of hybridization, the full range of apportionment of fuel and engine masses is considered, keeping the total take-off mass of the aircraft a constant. Different regimes are possible based on the power available from the engine to the battery relative to that required to propel the aircraft with take-off mass or that with empty mass in the conventional case. The range expressions are further different depending upon the times of complete fuel consumption or battery discharge, or time for full recharge of the battery in flight followed by the full consumption of the fuel, etc. The different regimes yield varying ranges that bridge the gap between the full-electric and full-combustion cases. An optimum apportionment of the fuel and engine mass exists for maximum range at each level of hybridization. Five different existing aircraft of vastly different passenger capacity from 4-150 are considered over the full spectrum of hybridization. The larger aircraft exhibit the likelihood of predominant combustion power to obtain reasonably high range, whereas smaller aircraft yield good range even with modest level of hybridization. © 2018, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.