The annual average rainfall in the coastal area of Bangladesh is more than 2,400mm and rainwater harvesting (RWH) has been practiced for a long time for drinking water supply. For household RWH, the capacity of the storage tanks varies from 1,000 to 5,000 litres, which are the model-type storage tanks provided under several RWH programs of government and NGOs. The optimum storage tank volume has not been investigated considering family size and demand, roof types and rainfall. As a result, most of the households can avail water from rainwater tanks for about six months and rest of the year, they have to depend on other unreliable and distance sources. With the aim of developing a comprehensive decision support tool for a reliable RWH system, design curves for the storage volume were developed for three climatic conditions (i.e. dry, average, and wet years), available roof catchment area (10-50m(2)), rainfall loss factor and household demand (2-12 lpcd) for a typical six members family using mass curve. Moreover, a spreadsheet-based daily water balance model was developed to assess the reliability of the currently used water tanks (1,000-5,000L). The analysis showed that the currently used tanks are insufficient to meet the year-long drinking and cooking water demand. Under average and dry climatic conditions, the achievable reliability does not significantly varies with increase of catchment area and tank size; and the maximum achievable reliability is about 70%. A large quantity of water is lost as spilled water even with a tank size of 5,000L. This water can be used for other purposes if larger tanks are used to capture the excessive spilled water.