The development of efficient gearboxes in electric drives is a major challenge in order to exploit the full potential of the electric powertrain and meet customer requirements in terms of range and efficiency. Precise efficiency prediction and consideration of tribological and geometrical influences are necessary to minimize losses in spur gears in electric drives. Thus, the aim of this work is to develop a calculation model for spur and helical gears that takes into account the effects of the microgeometry on the load distribution and corresponds to the latest state of the art. From this, a gearing-specific loss parameter, the local geometric tooth loss factor, can be derived that reflects the geometric influence of a gearing on its loss behavior. The thesis includes the fundamentals of calculating the power loss in a cylindrical gear pair, a mathematical description of the tooth profile and common gear modifications, and a method for calculating the load distribution using the influence number method. The presented calculation model is implemented as a MATLAB program and verified using commercial software and literature values of various test gears. The effect of microgeometry on loss behavior is investigated using a two-stage eDrive gearbox, where each stage has different gear geometry and load. It is observed that the application of profile crowning significantly relieves the load distribution areas of high sliding velocities, the beginning and end of gear meshing. Locally, lower losses occur and the efficiency behavior of a gear is significantly increased. Crowning as a flank line modification tends to have a negative effect on the loss behavior, since by relieving the end faces the load distribution is centered in the tooth center and locally high losses occur at the beginning and end of meshing. However, such modifications are necessary for load capacity reasons. By combining them, the advantages of both corrections on the efficiency and load capacity behavior can be united.