Increasingly frequent extreme weather phenomena, such as torrential rainfall, force the need to use retention and search for new technical solutions to minimize their negative effects, such as inundations or floods. One of the solutions are flow regulators that limit the flow rate from the catchment area and play an increasingly important role in water and wastewater management. Their main function is to ensure a constant value of the liquid outflow rate, as well as to stabilize the flows upstream and downstream of devices whose effectiveness depends on the volume of flow (wastewater pumping stations, pre-treatment devices). The answer to these needs is the “Flow regulator selection calculator”. The purpose of the tool is a quick and, at the same time, possibly precise selection of flow regulators for rainwater drainage systems.