Abstract: Building water mass balances were performed for one 150-story conventional building scenario for comparison with three scenarios of the 2020 Tower, a hypothetical 150-story high-rise building with on-site wastewater treatment and reuse. To ensure that the assumptions for the hypothetical building are appropriate, a one-year water balance was also conducted of the existing 27-story Solaire building that partly closes the water/wastewater loop, meters major water flows and implements low-flow/water conserving fixtures and appliances. For comparison, a conventional 27-story building scenario with the same low-flow/water conserving fixtures as the Solaire but no water reuse was also assessed. The mean daily indoor water use in the Solaire was 246 l/(d cap) which exceeds mean daily water use found in the literature. The water mass balances showed that an urban high-rise building needs another source of water even when potable reuse water is produced because of losses during water end use and treatment (i.e., evaporation, water in treatment residues). Therefore, water conservation (i.e., modification of human behavior) and water efficiency improvements (i.e., equipment, appliances and fixtures) are important major factors in reducing the municipal water needed in all scenarios.