Water supply is a challenging task to most city authorities. Dhaka, the capital of Bangladesh, has a population of 24 million, and does not have a reliable water supply system like developed countries. Although Bangladesh receives lots of rainfall (average rainfall of over 2000 mm), rainwater harvesting, which can meet a significant part of non-potable water demand, is not well-practiced like Australia. This paper presents a software application to demonstrate water savings via a rainwater harvesting system in a hypothetical 11-story building (with 40 apartments) in Dhaka city. It uses daily rainfall data and other model inputs such as roof area, water demand and loss factors to optimise the rainwater tank size for this building. It is found that 70 KL tank would be most appropriate for this building. This study shows the usefulness of rainwater harvesting software to support architects, engineers, residents and policy makers by providing a practical decision-making tool for RWH implementation.