The paper consider the basic principles of calculating the capacity of storm sewers of circular cross-section, which can operate both in pressure and gravity modes. The adequacy and limit conditions of application of simplified power-law equations for capacity assessing of sewers have been determined. The values of coefficients and power exponents of simplified equations for different pipe materials are given. It was determined that the values of the minimum allowable slopes of pipelines are practically the same for storm and combined sewers. Their values depend only on pipe diameters and can be calculated according to the obtained empirical equation. The equation parameters’ values for different pipe materials are determined on the basis of the proposed method of calculations. The values of the allowable minimum and maximum slopes, as well as the respective boundary values of the capacity of sewers of different diameters and pipe materials, were determined. The new approaches to assess the capacity of storm sewers are grounded on the universal method of hydraulic calculations of pipelines. It is based on the proposed simplified power-law equations, the obtained empirical relationships for the determination of the minimum and maximum allowable slopes and the respective capacities of sewers from different pipe materials and diameters. The effectiveness of performing hydraulic calculations of storm sewers according to simplified power-law equations is shown. It allows not only to simplify the calculations, but also to obtain based on them additional relationships between the structural and hydraulic parameters of the pipelines.