In the modern conditions of the development of science and technology, unmanned aerial vehicles (UAVs) of various types have gained unprecedented popularity for solving a variety of practical tasks. One of the most promising UAVs are electric multirotor UAVs. These UAVs have high potential for use in solving new practical tasks, so the development of theoretical and methodological foundations for their design is relevant. The main task of creating UAVs for any purpose is to form their quality, means, a set or combination of essential parameters, features, characteristics, indicators, properties, and attributes that ensure their high target efficiency. The quality level of UAVs is determined by their geometric shape, mass-inertial parameters (MIP), power unit characteristics, strength, elastic-geometric characteristics, and the perfection of automatic control system for the special task. The calculation of the takeoff mass of UAVs in the first approximation is carried out at the initial stages of design based on statistical data on built UAVs, which provides the first approximate information about their mass-inertial parameters, which correspond to certain flight-technical and other characteristics. Due to the novelty of the widespread use of electric multirotor UAVs, statistical data is insufficient, and the theoretical foundations of the calculation have not been established. Given this, the authors present such theoretical foundations for calculating the takeoff mass of electric multirotor UAVs in the first approximation, justify the form of the formulas, conduct practical verification, and obtain results which was comparing with real UAVs.