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Drain Pipe Sizing with DFU Tables (IPC 710.1)
What’s a DFU
A drainage fixture unit (DFU) is a load factor for how much a fixture is likely to discharge. You don’t add up gallons per minute, because not every fixture drains at once. You add up DFUs and the code tables turn that total into a pipe size.
Everything below is from the 2021 International Plumbing Code. The IRC’s plumbing chapters use similar but not identical tables, and some areas use the UPC instead. Use the code your jurisdiction adopted.
IPC Table 709.1: DFU Values
| Fixture | DFU |
|---|---|
| Lavatory | 1 |
| Bathtub (with or without shower over it) | 2 |
| Shower, 5.7 gpm total flow or less | 2 |
| Water closet, private, 1.6 gpf | 3 |
| Water closet, private, over 1.6 gpf | 4 |
| Kitchen sink, domestic (with or without disposer or dishwasher) | 2 |
| Dishwasher, domestic | 2 |
| Automatic clothes washer, residential | 2 |
| Laundry tray | 2 |
| Floor drain | 2 |
| Emergency floor drain | 0 |
| Bathroom group, 1.6 gpf water closet | 5 |
| Bathroom group, over 1.6 gpf water closet | 6 |
The bathroom group is the shortcut. A full bath (toilet, lavatory, tub or shower) with a 1.6 gpf toilet counts as 5 DFU, not 3 + 1 + 2 = 6. A half bath isn’t a bathroom group, so it’s 3 + 1 = 4.
IPC Table 710.1(2): Horizontal Branches and Stacks
| Pipe | Horizontal fixture branch | Stack, 3 branch intervals or less |
|---|---|---|
| 1-1/2” | 3 | 4 |
| 2” | 6 | 10 |
| 2-1/2” | 12 | 20 |
| 3” | 20 | 48 |
| 4” | 160 | 240 |
IPC Table 710.1(1): Building Drains and Sewers
| Pipe | 1/8” per ft | 1/4” per ft | 1/2” per ft |
|---|---|---|---|
| 2” | not allowed | 21 | 26 |
| 2-1/2” | not allowed | 24 | 31 |
| 3” | 36 | 42 | 50 |
| 4” | 180 | 216 | 250 |
| 6” | 700 | 840 | 1,000 |
Note a to this table: any building drain serving a water closet must be at least 3 inches. For converting between percent grade, inches per foot, and ratio, see the slope calculator guide.
A Typical House
Upstairs: two full baths with 1.6 gpf toilets (5 DFU each), laundry (2 DFU). Total: 12 DFU.
Downstairs: one full bath (5 DFU), one half bath (4 DFU), kitchen sink (2 DFU), dishwasher (2 DFU). Total: 13 DFU.
Building total: 25 DFU.
The building drain carries 25 DFU. A 3-inch building drain handles 36 at 1/8” per foot and 42 at 1/4”. A 3-inch drain works on paper. Plenty of designers and local rules use 4-inch anyway for the building drain and sewer, so check your local amendments.
The upstairs branch carrying 12 DFU? Table 710.1(2) allows 20 DFU on a 3-inch horizontal fixture branch. Because it serves water closets, it’s 3-inch at minimum regardless.
Slope
IPC Table 704.1 sets minimum slope for horizontal drain pipe:
- 2-1/2” and smaller: 1/4” per foot
- 3” to 6”: 1/8” per foot
- 8” and larger: 1/16” per foot
The IRC uses the same split (Section P3005.3). A 3-inch line running 20 feet needs at least 2-1/2 inches of fall at 1/8” per foot. A 2-inch line on the same run needs 5 inches.
This is where drain layout and framing collide. In a 2x10 floor, a 20-foot 2-inch run eats 5 inches of the joist depth plus the pipe. You may need to run perpendicular to the joists through holes that follow the IRC notching and boring limits, or drop a soffit.
Vents
Vents are part of the same system. Individual, branch, and circuit vents must be at least half the diameter of the drain they serve and never smaller than 1-1/4 inches. Over 40 feet of developed length, go up one pipe size (IPC 906.2). Stack vents and vent stacks are sized from IPC Table 906.1 by stack size, DFU load, and developed length.
Undersized vents cause slow drains, gurgling, and siphoned traps, even when the drain itself is sized right.
Commercial Work
Commercial drainage uses the same method with more fixture types: public water closets (4 DFU at 1.6 gpf), urinals, floor sinks, grease interceptors, and fixtures not in the table that are rated by outlet size (IPC 709.2). Most commercial jobs are designed by an engineer and drawn on the plans.
Running It on Your Phone
Trussly’s DFU and Drain Pipe Sizing calculators let you pick fixtures, total the DFUs, and size branches, stacks, and the building drain. They use the IRC’s fixture values and tables, since that’s the code most houses are built under, and they show the section next to each result. Switch to the IPC tables above if your jurisdiction uses the IPC for one- and two-family homes.