A drainage fixture unit (DFU) is a load value the plumbing code assigns to each fixture. You add up the DFUs feeding a pipe, then read the minimum pipe size for that total and slope from the code's sizing table. The IPC and UPC both work this way, and toilets carry the biggest values.
What a drainage fixture unit is
A drainage fixture unit, or DFU, is a rating, not a flow you can measure with a bucket. The International Plumbing Code (IPC) says its values "designate the relative load weight of different kinds of fixtures" for estimating the total load on a soil or waste pipe (Section 709.1). A lavatory is a small number. A public toilet is several times that.
The idea behind fixture units is probability. NIST explains it on the water-supply side: "Since it would not be expected that all fixtures in a building would be used at the same time, the curves incorporate the probability of use." A drain sized for every fixture flushing at once would be far bigger than most buildings need, so the tables assume a realistic mix.
How the IPC and UPC assign values
IPC. Table 709.1 lists a value for each fixture type, and for groups such as a bathroom group. For fixtures not listed, Sections 709.2 through 709.4.1 give methods to compute a value, including for devices with intermittent flow. One rule matters for low-flow toilets: for building drains and sewers, the IPC says water closets and urinals "shall not be rated at a lower drainage fixture unit unless the lower values are confirmed by testing." In other words, the drain load is not automatically reduced because a toilet is water-saving.
UPC. The Uniform Plumbing Code (UPC) ties the unit value to the trap size (Section 702.1), and its Table 702.1 lists separate columns for private, public and assembly use. It also covers flows that aren't in the fixture table. Per Table 702.2, intermittent discharge up to 7 1/2 gallons per minute equals one fixture unit, and Section 702.3 counts continuous flow, such as a pump, sump ejector or air conditioning equipment, as 2 fixture units for each gallon per minute.
How the total picks the pipe size
The IPC's Section 710.1 says the maximum DFUs on a building sewer, building drain or horizontal branch of the building drain come from Table 710.1(1). Branches and stacks use Table 710.1(2). The building drain table is organized by pipe diameter and slope per foot, so the same pipe carries fewer units when it runs flatter. A note under Table 710.1(1) sets a floor: "The minimum size of any building drain serving a water closet shall be 3 inches."
The UPC does the same job in Section 703. Minimum sizes "shall be determined from the total of fixture units connected thereto," and for vertical pipes also by length (703.1). Table 703.2 gives the maximums, and a footnote reduces horizontal capacity when the pipe is laid at the flatter 1/8 inch per foot. See our page on drain pipe slope by code for why slope matters that much.
We're not reproducing the tables here. They are long, they differ by code and edition, and the number that counts is the one in the code your plan reviewer uses.
What Phoenix adds
Phoenix publishes its amendments as two documents, one to the 2024 IPC and one to the 2024 UPC, adopted by Ordinance G-7397 effective August 1, 2025. Two of them touch drain sizing and slope. In the IPC amendment, piping upstream of a grease interceptor must have a slope of not less than 1/4 inch per foot. In Phoenix's version of Section 718.1, an exception for flatter building sewers ends with: "The maximum and minimum fixture unit loading shall be in accordance with Table 717.1."
A second exception in both documents lets the city approve a flatter slope instead of a sewage ejector or pumping station, but only when a registered engineer or architect certifies the design and installation and the owner agrees in writing, under notary, to accept it. That is a case where a design professional is required.
Adding fixtures to an existing building
The IPC's Section 102.4 says additions and alterations "shall not cause an existing system to become unsafe, unsanitary or overloaded." Phoenix's amendment leaves that section as written. In practice that means adding a restroom group, a commercial dishwasher or a floor sink means adding its DFUs to what the existing drain already carries, and checking the total against the table. Phoenix requires a commercial building permit for non-residential remodels, additions and tenant improvements, and says the permit is issued when the design meets the building codes.
If the existing line is short on capacity or the slope is marginal, you want to know before the permit drawing, and a sewer camera inspection shows what you're working with. HQ handles commercial drain and sewer plumbing, is licensed and insured, and gives free estimates with upfront pricing. Also see how many fixtures a commercial space needs and what a commercial tenant improvement involves.