When determining the necessary capacity of a drainage or sewer system, it is vital to know the flow of material that is going to be traveling through it. In plumbing terms, flow is typically expressed in gallons per minute. If you don't have the time or the equipment to accurately measure the average number of gallons of material that are flowing through your system at a given time, however, you can calculate it by adding up the total number of drainage fixture units of the various devices that are hooked up to the system.
Locate the Uniform Plumbing Code (UPC) for your state. Copies of it are available from the International Association of Plumbing and Mechanical Officials and the group's website, Iapmo.org.
Locate the drainage fixture unit rating for your device or system in the UPC.
Divide the drainage fixture unit rating by 2 to get the gallons per minute of flow for your device or system.