The aim of the following paper is to evaluate methods of water meter selection and diameters of sewer connections currently used in designing in the light of measurements of water actual consumption in water supply system in selected building. The object of the research is multi-family residential building situated in northern Krakow. The scope of work includes the analysis of water consumption per hour and per second by residents of the building and verification of the correct selection of currently functioning water meter and diameter of sewer connection. Three methods used for the selection of these two elements of water supply system in Krakow based on the actual measurements of water consumption have been evaluated. The research has shown that none of the analysed methods of water meter diameter selection as well as sewer connection allow to achieve results completely consistent with the actual flow rate of water consumption. Observed and registered actual water flows in the building have been up to 15 times lower than some design flows. Oversizing of both water meter diameter and sewer connection diameter has been found for each of the analysed measurement methods. A significant discrepancy between measured and empirical parameters indicates the need to improve measurement methods for the selection of water meters and diameters of sewer connections.