Purpose: The normative and methodological documents in Russia regarding the treatment of surface runoff from residential areas do not provide explanations on the effectiveness of the treatment system from the point of view of the possibility of pollution of water bodies they allow. The probability of pollution and the dilution ratio of untreated wastewater have not been studied. The goal is to develop a method for determining the hydrological characteristics of the runoff necessary for the probabilistic assessment of dilution factors, permissible concentrations and discharges of contaminated surface waters. Methods: Based on observational data from fourteen weather stations in the North-West of Russia, the probabilistic relationship between the characteristics of runoff from built-up areas and the receiving river has been studied, taking into account the basics of probability theory and hydrological calculations of minimum runoff. Calculations have been carried out and the dependence of the number of days with precipitation and their relative amount in the warm period of the year on the maximum daily precipitation layer has been established. A mathematical relationship has been found between the frequency of rain and the number of possible cases of pollution of a water body. Results: A method has been developed for calculating the probable values of precipitation characteristics, minimum water runoff in the receiving river and the dilution factor of pollutants depending on the permissible frequency of negative consequences from river water pollution and the correlation coefficient of the probability of minimum water flow from built-up areas and in the receiving river. Practical significance: The method for calculating the hydrological characteristics of runoff will make it possible to more reasonably and accurately determine the required degree of wastewater treatment, the productivity and parameters of treatment facilities, and in some cases, to reduce the degree of pollution of watercourses.