To calculate using average hourly demand method, count the fixtures, multiply the number of fixtures by the gallons per hour (GPH) for the fixture in the particular type of building, and add them. Then multiply this total by the simultaneous usage factor to get the maximum hourly demand for the system. The minimum recommended storage volume then is calculated by multiplying the total demand by the storage factor. This is a simple method, but it has limitations:

• It can be applied only to the types of facilities listed.
• It can be used only for the sizing of storage tank systems.
• It does not consider the types of occupants.