There is a growing interest in using reservoirs for the economic use of rainwater for watering gardens. The tank is to capture as well as temporarily hold rainwater from the drain pipe, cleaned of solids thanks to the filter. The size of the tank depends on the roof surface from which rainwater is drained, reduced by the runoff factor. The tank should be equipped with an emergency overflow, which will drain excess water. The methods used to calculate the capacity of a rainwater tank so-far (so-called tank size calculators) are too simplistic and do not take into account the way of irrigation and the type of soil. The article presents a more accurate way of determining the demand for water for watering the garden according to the German standard DIN 1989-1: 2002-04.