A plumbing fixture unit is a number the code gives each fixture to rate its load on the pipes. Drainage fixture units (DFU) size drains and vents; water supply fixture units (WSFU) size water lines. A lavatory is 1 DFU; a public 1.6 gpf toilet is 4. Add them up and read the pipe size from a code table.
What exactly does a fixture unit measure?
Probable load, not actual gallons. The 2024 International Plumbing Code defines the drainage fixture unit as "A measure of the probable discharge into the drainage system by various types of plumbing fixtures." It goes on: "The drainage fixture-unit value for a particular fixture depends on its volume rate of drainage discharge, on the time duration of a single drainage operation and on the average time between successive operations."
That last part matters. Fixture units build in the fact that fixtures don't all run at once. Fixture-unit methods account for probable demand rather than simply multiplying every fixture's maximum flow by the fixture count. Actual simultaneous use can vary; the designer still has to follow the applicable sizing method and account for special equipment or operating conditions. That's why a busy public toilet rates higher than the same toilet in a home: people use it more often.
The water side works the same way. Appendix E of the IPC says the water supply fixture unit "measures the load-producing effect of a single plumbing fixture of a given kind," and that the units "are then converted into gallons per minute (L/m) flow rate for estimating demand."
What are common drainage fixture unit values?
Here are values from IPC Table 709.1, used in Phoenix for projects designed to the IPC (Phoenix allows either the IPC or the UPC):
| Fixture | DFU |
|---|---|
| Lavatory | 1 |
| Drinking fountain | 1/2 |
| Bathtub (with or without shower) | 2 |
| Kitchen sink, domestic (with or without disposer or dishwasher) | 2 |
| Residential clothes washer | 2 |
| Commercial clothes washer | 3 |
| Floor drain | 2 |
| Service sink | 2 |
| Water closet, private, 1.6 gpf | 3 |
| Water closet, public, 1.6 gpf | 4 |
| Water closet, public, flushing more than 1.6 gpf | 6 |
| Urinal | 4 |
| Urinal, 1 gallon per flush or less | 2 |
| Urinal, nonwater supplied | 1/2 |
| Bathroom group, 1.6 gpf toilet | 5 |
A bathroom group is a toilet, lavatory, and tub or shower in the same room, rated together. It rates lower than the three fixtures added one by one, because they're rarely all draining at once.
Two rules from the table's notes are worth knowing. "For fixtures added to a bathroom group, add the dfu value of those additional fixtures to the bathroom group fixture count." And "For the purpose of computing loads on building drains and sewers, water closets and urinals shall not be rated at a lower drainage fixture unit unless the lower values are confirmed by testing." The table already rates a 1.6 gpf toilet lower than an older, larger-flush one, but when sizing the building drain and sewer, a toilet or urinal can't be rated any lower than its table value without test data.
### What about fixtures that aren't in the table?
The code gives two fallbacks. For an unlisted fixture, Section 709.2 rates it by the size of its drain outlet using Table 709.2. For equipment that discharges a steady flow, Section 709.3 says "1 gpm (0.06 L/s) of flow is equivalent to two drainage fixture units." So a piece of equipment that drains 3 gallons per minute counts as 6 DFU.
How do fixture units size a drain pipe?
You add the DFUs flowing through each section of pipe, then look up the smallest pipe that can carry that total at its slope. The IPC has two main tables.
Horizontal fixture branches (Table 710.1(2)). These are the drains that collect fixtures on an upper floor and carry them to a stack. The table's note says it "Does not include branches of the building drain. Refer to Table 710.1(1)." So a restroom line in a slab on grade, the usual Valley office or shop, is sized from the building drain table below.
| Pipe size | Max DFU on a horizontal branch |
|---|---|
| 1-1/2 inch | 3 |
| 2 inch | 6 |
| 3 inch | 20 |
| 4 inch | 160 |
Building drains and sewers (Table 710.1(1)), at 1/4 inch per foot of slope:
| Pipe size | Max DFU |
|---|---|
| 3 inch | 42 |
| 4 inch | 216 |
| 6 inch | 840 |
Slope changes the answer. The same 4-inch building drain carries 180 DFU at 1/8 inch per foot and 250 at 1/2 inch per foot. The table also has a floor: "The minimum size of any building drain serving a water closet shall be 3 inches."
### A worked example
A small office restroom on the second floor, draining to a stack, has two public 1.6 gpf toilets, one 1 gpf urinal, two lavatories, and a floor drain:
- 2 toilets × 4 = 8 DFU
- 1 urinal × 2 = 2 DFU
- 2 lavatories × 1 = 2 DFU
- 1 floor drain × 2 = 2 DFU
- Total: 14 DFU
That's under the 20 DFU a 3-inch horizontal branch can carry. Add a second restroom of the same size on that floor, and the combined 28 DFU goes past what a 3-inch fixture branch carries, so the shared branch has to go up to 4 inch. Put the same two restrooms on a slab instead, and they tie into a branch of the building drain, where a 3-inch line carries 42 DFU at 1/4 inch per foot. There, 28 DFU stays at 3 inch. Real designs also have to check each stack, the vents, and the building drain. That's the plumbing designer's job on the permit drawings.
How do water supply fixture units work?
The same idea, different numbers. Appendix E, Table E103.3(2), gives each fixture a WSFU value, often split into cold, hot, and total. Some totals:
| Fixture | WSFU (total) |
|---|---|
| Lavatory, private | 0.7 |
| Lavatory, public | 2.0 |
| Kitchen sink, private | 1.4 |
| Kitchen sink, hotel or restaurant | 4.0 |
| Water closet, private, flush tank | 2.2 |
| Water closet, public, flush tank | 5.0 |
| Water closet, public, flushometer valve | 10.0 |
| Bathroom group, private, flush tank | 3.6 |
The designer adds the WSFUs on each water line, converts the total to a gallons-per-minute demand using the code's curve, then sizes the pipe for that flow and the pressure available. The appendix gives an example: "a total of 120 water supply fixture units is converted to a demand of 48 gallons per minute."
One catch: fixture units only work for fixtures used on and off. The appendix says "Fixture units cannot be applied to constant use fixtures such as hose bibbs, lawn sprinklers and air conditioners." Those get their real gallons per minute added on top of the fixture-unit demand. The same example adds two hose bibbs at 5 gpm each, for 58 gpm in total.
Why do fixture units matter to building owners?
Because they decide whether a building's pipes can take more fixtures. Fixture units come up when:
- You add a restroom or a kitchen in a tenant space. The existing branch, stack, or building drain may already be near its DFU limit.
- You convert a use, such as retail to restaurant. The fixture count and the ratings both go up.
- Drains back up at peak times. An undersized line can work fine most of the day and overflow when everything runs at once.
- You size a water meter or service line. More WSFUs mean more demand and possibly a larger service.
For the one-line definition, see our glossary entry on the drainage fixture unit. For a related sizing question in commercial kitchens, see what size grease interceptor a restaurant needs.
The values above come from the 2024 IPC. Other codes, like the Uniform Plumbing Code, use their own fixture unit tables, and a few Valley cities are on older IPC editions, so check the code your city has adopted.
HQ Plumbing & Air plans and installs drain, waste, vent, and water piping for commercial build-outs and tenant improvements across the Phoenix area, and can check whether existing lines can carry added fixtures. We're licensed and insured, with free estimates and upfront pricing. See our commercial plumbing services.
Add up the units, read the table, size the pipe. That's the whole system.