Water Resour Manage (2010) 24:2381–2403 DOI 10.1007/s11269-009-9557-4 Parameters Influencing the Sizing of Rainwater Tanks for Use in Houses Enedir Ghisi Received: 19 November 2008 / Accepted: 6 December 2009 / Published online: 18 December 2009 © Springer Science+Business Media B.V. 2009 Abstract Rainwater harvesting has been studied in different countries as a way of easing water availability problems and reducing potable water demand in buildings. The most important factor relating to the efficiency of a rainwater system is the cor- rect sizing of the rainwater tank. Therefore, the objective of this article is to assess the influence of rainfall, roof area, number of residents, potable water demand and rain- water demand on rainwater tank sizing. The analysis was performed by using com- puter simulation and by considering daily rainfall data for three cities located in the state of São Paulo, Brazil. The roof areas considered were 50, 100, 200 and 400 m 2 ; the potable water demands were 50, 100, 150, 200, 250 and 300 l per capita per day; the rainwater demands were taken as a percentage of the potable water demand, i.e., 10% to 100% at increments of 10%; and the number of residents was two and four. Results indicated a wide variation of rainwater tank sizes for each city and also for each parameter. The main conclusion that can be made from the study is that rainwater tank sizing for houses must be performed for each specific situation, i.e., considering local rainfall, roof area, potable water demand, rainwater demand and number of residents. Therefore, sizing rainwater tanks according to local tradition is not recommended as it may incur low efficiency. Keywords Rainwater tank sizing · Computer simulations · Houses · Sustainable development 1 Introduction Rainwater harvesting, amongst other strategies, has been studied in different coun- tries as a way of easing water availability problems. It can be found in the literature E. Ghisi (B ) Laboratory of Energy Efficiency in Buildings, Department of Civil Engineering, Federal University of Santa Catarina, Florianópolis, SC, 88040-900, Brazil e-mail: enedir@labeee.ufsc.br