Civilization progress entails an increasing demand for water. There are many different technologies for collecting water from alternative sources. An efficient and cost-effective solution is to use small tanks for rainwater runoff from rooftops. This way the harvested water can be successfully used to flush toilets, which stands for 30 percent of water consumption in households. The capacity of water tanks depends on technical and economic possibilities. Using a dual flush system in toilets that allows for both rainwater and mains water usage can lead to a significant reduction in tank size. In Bielsko-Biała, given its environment, one-family dwellings, a rainwater tank with a capacity of only 0.25 m3 per person and a roof area of 30 m2 per person, it is possible to save almost 80 percent of treated mains water used in toilet flushing. It constitutes over 10 m3 of water per person per year. A nomogram made for choosing the right tank according to roof area and tank capacity allows to assess the possibilities in terms of drinking water savings in any building in the environment of Bielsko-Biała.