Rainwater harvesting (RWH) is one solution that can be implemented in urban areas to increase the availability of raw water and reduce flood losses. The objectives of this study is to obtain design curve for designing RWH reservoir according to catchment area, required water demand and expected reliability. The study is located in Jakarta Region for 4 sub region area. The design curve shows that increase in water demand and expected level of reliability, will be followed by increment of reservoir volume. RWH will be utilized as alternative source in Jakarta therefore 70% of reliability level is recommended. Building with a catchment area larger than 140 m2 is recommended to choose 2 m3 reservoir while smaller building choose 1 m3 reservoir. 2 m3 reservoir can serve maximum 240 L / house / day while 1 m3 serve maximum of 120 L / house / day.