Many homes use pressurized diaphragm tanks for storage of water pumped from an underground well. These tanks are very carefully constructed to have separate internal chambers for the storage of water and for the air that provides the pressure. One might expect that the amount of water available for use from, for example, a 50-gallon tank would be close to 50 gallons. However, only a surprisingly small percentage of the total tank volume is available to provide water that can be drawn from the tank before the pump must cycle back on. Boyle's law (PV is constant) provides mathematical insight into the workings of this type of tank, including predictions of the quantities of available water resulting from different initial conditions of the water tank system. (Contains 5 figures and 1 table.)