The operating cost calculator, whose authors are practitioners working in three companies, Veolia, DSConsulting and RetencjaPL, with comprehensive knowledge, experience and data in the field of shaping, using and financing stormwater infrastructure, allows for a quick and, at the same time, possibly precise calculation of the costs of maintaining drainage and water retention systems rainwater along with the calculation of the rate of potential charges for rainwater. This is the first calculator addressed to managers of rainwater drainage systems: waterworks, municipal companies, city authorities interested in making the conditions for maintaining rainwater systems more realistic or introducing charges for rainwater and assessing the conditions in which this process should be carried out.