Considering the increasing salinity intrusion in coastal areas, pollution of surface water bodies, groundwater contamination by arsenic and manganese, and groundwater depletion due to excessive withdrawal, rain water harvesting (RWH) has become a potential source of water supply in coastal and arsenic affected area in Bangladesh. A user-friendly software using a simulation model has been developed and employed to estimate the optimum rainwater storage tank size, which is the most costly component of a RWH system. The procedure developed constitutes an effective tool for estimation of the most satisfactory storage capacity for any combination of location, catchment area and material, and water demand. The software also estimates reliability of the corresponding water supply system. The runoff coefficient for various types of materials and the rooftop area for the concerned household are considered as variables. The rainfall data for a 24-year period for different areas of Bangladesh was collected from Bangladesh Meteorological Department (BMD) and used in the model. Yield After Spillage (YAS) concept was used to develop the program, which provides estimation on the actual rainwater availability and storage conditions. Volumetric reliability index was employed to find the optimum size of the rooftop rain water harvesting (RWHS) tank size, and the corresponding time reliability was determined. Volumetric reliability has been determined considering water demand for drinking and cooking purposes only. The software will provide optimum tank size in terms of size and efficiency along with the approximate functional period for any location in Bangladesh for any particular family size and roof catchment. The software was employed to assess RWHS in a coastal area (Khulna), in an arsenic affected area (Comilla) and a low-precipitation area (Rajshahi). The software would be very useful in determining optimum tank size of RWHS and estimating operational period of such systems for any area of Bangladesh.