This study demonstrates a large roof (30,000 m2) rainwater harvesting (RWH) system in an indoor arena by considering three water demand scenarios (toilet flushing, irrigation and combined demand) via hydraulic and economic assessments. The water saving efficiency (WSE) of the RWH system for each scenario was estimated by a simulation model using historical daily rainfall data (1968–2018). Depending on the water demand, the WSE was found to be independent of tank size when the tank size exceeded 1000 m3. The results suggest that the WSE of the RWH system is highly influenced by water demand scenarios, and a storage capacity of 400–1000 m3 would be enough for the applications considered in this study. The economic analysis results further showed that depending on the water demand, the RWH system with a rainwater storage capacity of between 100 and 600 m3 was more economically beneficial due to its positive cost saving values. The results also showed that depending on the water scenarios, the unit water cost between 0.37 and 0.40 £/m3 was lower than the mains water cost (0.40 £/m3). As a result, the use of the RWH system with a tank between 400 and 600 m3 can be the most favourable range under the conditions considered in this study. Given the variations in water price, rainfall patterns and discount rates, the sensitivity analysis showed that water tariffs and discount rates play a significant role in reducing the unit water cost of the system, maintaining it lower than the mains water cost. A payback period analysis of the RWH system with a 600 m3 tank revealed that a 5% discount rate and a water price of 3 £/m3 would be enough to make the RWH system cost effective and that the capital cost could be returned within 10–11 years. This study highlights the need for preliminary sizing of a rainwater tank and an economic analysis of a large rooftop RWH system to maximise the benefits.