Fixture Unit (DFU)
A fixture unit is a code unit of measure that rates how much water a fixture supplies or drains, letting designers size pipes and vents for the combined demand of a building.
Plumbing code cannot size pipes by guessing, so it assigns each fixture a number. On the drainage side these are drainage fixture units (DFU): a toilet might count as several DFU, a lavatory sink for one, and a tub for two. Adding up the fixture units served by a pipe tells the designer how large that drain or vent must be to carry the peak load without backing up.
The same idea, with water supply fixture units, sizes the supply pipes so that several fixtures running at once still get adequate pressure and flow. This is why a single bathroom can run on a smaller branch while a whole house needs a larger main. When you add a bathroom, the new fixtures add fixture units, and the existing drains and vents must be able to handle the higher total or be upsized.
Homeowners rarely calculate fixture units themselves, but the concept explains why a plumber may say an existing drain or vent is too small for added fixtures. It is the math that keeps a system from being overloaded.
Talking to a contractor about this?
Run the project past a licensed plumbing pro first. Calls are answered around the clock and routed to a pro serving your area.
- Plumbing Permit : A plumbing permit is official authorization from a local building department to perform plumbing work, paired with one or more inspections to confirm the work meets code.
- Backflow Preventer : A backflow preventer is a device that keeps water from flowing backward through the plumbing, protecting the clean supply from contamination if pressure drops or reverses.
- Cross-Connection : A cross-connection is any point where the clean drinking water supply can come into contact with a non-potable source, creating a path for contamination to enter the potable system.
- Licensed Plumber (Journeyman & Master) : A licensed plumber is a tradesperson who has met state or local requirements to perform plumbing work legally, typically ranked as a journeyman who works independently or a master who can design systems and pull permits.