What fixture unit means
A fixture unit is a number the code assigns to each plumbing fixture to estimate its load. Drainage fixture units size drain and vent pipes; water supply fixture units size supply pipes.
Also called DFU, drainage fixture unit, WSFU.
Where it matters on a plumbing job
Adding a bathroom means adding fixture units to an existing drain and supply. Do the count before you promise the existing lines will carry it, and note it on the permit drawings.
What the model code says
- Drain, waste and vent piping loads are computed in drainage fixture unit values from the code's table. (IRC 2021, Chapter 30)
Check which code edition your building department enforces for fixture unit before you quote.
What it costs
Koira's bathroom remodel cost guide puts a typical job at $12,000, with most between $6,250 to $18,000. The guide shows its sources and what moves the price. The water pipe sizing calculator prices a job line by line.
Work it out
- Free calculator: Water pipe sizing calculator
- Cost guide: Bathroom remodel cost
- Licensing: Plumbing license rules by state
- Tools, templates and guides: Free plumbing resources
Sources: 2021 International Residential Code, Chapter 30, Sanitary Drainage (ICC). Checked October 9, 2026.