Fixture units (DFU)
Also known as: DFU, WSFU, drainage fixture units
Standardized measure of expected water demand from plumbing fixtures. Used to size drain and supply piping per the plumbing code.
Fixture units are a standardized way of expressing the expected water demand of various plumbing fixtures. Drainage fixture units (DFU) measure expected drainage flow; water supply fixture units (WSFU) measure expected supply demand.
For reference (DFU values): a residential lavatory = 1, kitchen sink = 2, bathtub or shower = 2, water closet (toilet) = 3-6 depending on flush volume, residential clothes washer = 2, dishwasher = 2. Total fixture units for a building determine the required drain pipe size (e.g., a 3-inch drain handles up to 35 DFU; a 4-inch handles 256). Supply sizing follows similar principles.
The practical implication for plumbers: any addition to a building (new bathroom, additional sink, finished basement plumbing) requires re-checking that existing drain and supply lines are adequate. Adding fixtures beyond the rated capacity of existing pipe is a code violation and creates real performance problems (slow drains, pressure drops). Knowing the DFU table from memory is foundational plumbing-code competency.
Related terms
P-trap
U-shaped pipe under sinks and fixtures that retains water to block sewer gas from entering the building. Required by plumbing code on every drain.
Backflow preventer
Plumbing device that prevents contaminated water from flowing backward into the potable water supply. Required by code on irrigation, fire suppression, and many commercial connections.