A water tank is a container for storing water. Water tanks are used to provide storage of water for use in many applications, drinking water, irrigation agriculture, fire suppression, agricultural farming, both for plants and livestock, chemical manufacturing, food preparation as well as many other uses. Water tank parameters include the general design of the tank, and choice of construction materials, linings. Various materials are used for making a water tank: plastics (polyethylene, polypropylene), fiberglass, concrete, stone, steel (welded or bolted, carbon, or stainless). Water tanks are an efficient way to help developing countries to store clean water.
This video about how to design water tank for 4 family members, volume of water tank and its size with water capacity , you can use the same formula to find size of rectangular water tank, but only you can change your family members Numbers.