Large scale rainwater harvesting (LSRWH) is a promising alternative to address security and flood issues in urban areas. The development and planning of a LSRWH for an eco-community involves comprehensive site-planning, systems planning and design, which includes the rainwater source collection (roof) to the storage tank, the distribution and allocation system. This paper describes a new technique for designing a LSRWH for a community of 200 houses with an average of 4 persons per house and an average total daily water demand of 160 m(3). It was found that the optimal size storage tank for a 20,000 m(2) roof area is 160m(3) with a 60% reliability. The application of this model to the case study revealed a significant water saving up to 58%. The total cost for this system is 443,861 USD over a life-span of 25 years. (C) 2013 Elsevier B.V. All rights reserved.