Rainwater harvesting is an ancient practice that involves collecting, storing, and using precipitation to meet on-site water needs. This paper develops and demonstrates guidelines for sizing the capacity of storage tanks to provide a reliable continuous supply of harvested rainwater for residential households. Operation of the rainwater harvesting system is simulated with a stochastic mass balance performed on an Excel spreadsheet. The daily volume of rainwater in an unbounded tank is tracked to determine the maximum accumulated deficit on a monthly basis. Results are summarized in dimensionless charts showing the minimum size of a rainwater storage tank needed to meet water demands at specified levels of reliability. (C) 2015 The Authors. Published by Elsevier Ltd.