Every sizing decision in plumbing starts the same way: count the fixtures, get the units. Fixture units are the currency — this is what they are and how the count works.
What a fixture unit is
A fixture unit is a standardized load rating — not gallons, not pressure, a number that represents how much demand one fixture puts on the system. One fixture unit is roughly the load of a lavatory faucet. Everything else is rated against it.
Two kinds matter:
- WSFU — water supply fixture units. Demand on the supply side. Used to size service, mains, and branches.
- DFU — drainage fixture units. Load on the drain side. Used to size branches, stacks, and building drains.
Same fixture, different numbers on each side — a water closet might carry 2.5 WSFU but 4 DFU because draining it is the bigger load.
Typical values (published code tables)
Approximate values from the public UPC/IPC-style tables for private residential use:
- Lavatory — 1.0 WSFU / 1.0 DFU
- Kitchen sink — 1.5 / 2.0
- Dishwasher — 1.5 / 2.0
- Clothes washer — 1.5 / 3.0
- Shower — 2.0 / 2.0
- Bathtub/shower combo — 2.0 / 2.0 (often rated by group)
- Water closet, tank type — 2.5 / 4.0
- Hose bib — 2.5 / 0
- Bathroom group (WC + lav + tub/shower) — ~5.0 / ~6.0
Your code book and your AHJ are the authority — these are the commonly published values, not the last word.
Why the total matters
The whole-home WSFU total tells you the class of service the house needs before anyone draws a line:
- Under ~12 WSFU — ¾" class service
- Under ~24 — 1" class
- Under ~48 — 1¼" class
- Under ~96 — 1½" class
Same idea on the drain side: DFU totals set branch and stack sizes. That's why fixture-unit calculators exist — the count is fast, and the size class follows from published tables.
Where the simple count stops
Totals give you the class. Real sizing goes branch by branch: which fixtures share a run, how long the run is, what pressure is available, what fittings eat head along the way. Two houses with the same WSFU total can need different branch sizes because the layouts differ.
Inside NectARM, the plumbing calculator counts fixture units off the drawing itself — place fixtures on the isometric and the system tracks demand per branch, sizes the runs, and feeds the takeoff. The free demo does the public-table count; the branch work stays inside.