Most of the population in Indonesia uses groundwater to meet the need for clean water. With the rapid population growth, there has been an increase in the conversion of open space land into residential, offices, and commercial areas. The land conversion increases the impervious surface throughout the city and could impact the availability of clean water in the future. Implementation of water conservation is needed to ensure long term availability of clean water. Rainwater harvesting systems are one of the methods to utilize rainwater as an alternate clean water source and water conservation. This study aims to calculate the volume of rainwater harvested in mall building in South Tangerang using EPA-SWMM to design a rainwater reservoir. The result of this study is an average of 80 m3/day of rainwater harvested from mall building in South Tangerang. The design for the reservoir is rectangular with dimensions of H 4 m × W 18.3 m × W 6.5 m and a capacity of 476.06 m3.