Each plumbing fixture has a fixture unit counts (FU) which can be obtained from the Uniform Plumbing Code (UPC) at Domestic water chapter (attached table).
By multiplying the number of each similar plumbing fixture to FU for that fixture from UPC table and adding all the total FU, the grand total of FU can be estimated. By referring to the FU-gpm curves in UPC can find the equal gpm for the grand total FU which would be the domestic water use for the building (attached curves.
