I had to Google "fixture units". No simple answer found.
"Drainage Fixture Units
A DFU is a measurement unit for potential water use for a given plumbing fixture or appliance, as provided for in the Building Officials & Code Administrators (BOCA) National Plumbing Code. "
A water closet of 1.6 GPF or less gets a DFU of 3.0 in Virginia Beach, where the DFU sum is computed as part of an inspection fee. A bathtub is 4.0 DFUs.
A water closet of 1.6 GPF gets a Water Supply Fixture Unit of 2.5 in Seattle, where they are used for sizing pipes. A bathtub is 10.0 WSFUs.
Apparently WSFUs are not the same as DFUs. It's not clear if they're the same everywhere, or how they are actually derived.
Maybe the cited code will provide a clue, but it's clear that there isn't going to be a simple correlation between FUs and gallons, because the FU is time- averaged and adjusted for duty cycle of each individual water sink or drainage source, and I'd bet there's been some political input toward encouraging use of low volume water closets, which are big water wasters, especially if there's a lot of fiber in your diet.
Mike Halloran
Pembroke Pines, FL, USA