As households are being more energy aware over the years, the energy consumption for providing domestic hot water (DHW) is receiving increasing attention. Thus, the design of hot water systems is becoming more important to ensure a holistic approach to energy reduction. Current sizing for these systems is often based on empirical practice. An inaccurate evaluation of the hot water demand could lead to a poor hot water system design. This either means an insufficient amount of hot water available to occupants or an oversized system that never gets to be used under its optimal operational point. Therefore, it is crucial to properly evaluate the hot water demand when designing hot water systems for dwellings. As an attempt to help engineers in the design process, a stochastic tool was constructed based on validated published tools to generate hot water demand profiles for residential buildings. This novel tool has been validated using real data from an apartment building in Quebec City, Canada. This building includes 40 apartments where hot water demand was monitored. A comparison between the monitored and simulated hot water demands of the building shows great accuracy of the tool. An improved sizing method based on the DHW profile generation tool is proposed and compared to traditional sizing methods.