Building Drain & Sewer Capacity Chart — DFU by Size and Slope
Maximum drainage fixture units on any portion of the building drain or building sewer, by pipe diameter and slope. The IPC prints a four-slope matrix — 1/16", 1/8", 1/4", and 1/2" per foot — while the UPC prints single horizontal capacities at a 1/4"-per-foot basis with a multiplier for flatter runs; the two families are never interchangeable. IPC values per Table 710.1(1) (2021 edition; unchanged 2012–2024); UPC values per Table 703.2 (2021 edition).
IPC — building drain & sewer capacity by slope
| Pipe size | 1/16" per ft | 1/8" per ft | 1/4" per ft | 1/2" per ft |
|---|---|---|---|---|
| 1-1/4" | — | — | 1 | 1 |
| 1-1/2" | — | — | 3 | 3 |
| 2" | — | — | 21 | 26 |
| 2-1/2" | — | — | 24 | 31 |
| 3" | — | 36 | 42 | 50 |
| 4" | — | 180 | 216 | 250 |
| 5" | — | 390 | 480 | 575 |
| 6" | — | 700 | 840 | 1,000 |
| 8" | 1,400 | 1,600 | 1,920 | 2,300 |
| 10" | 2,500 | 2,900 | 3,500 | 4,200 |
| 12" | 3,900 | 4,600 | 5,600 | 6,700 |
| 15" | 7,000 | 8,300 | 10,000 | 12,000 |
UPC — horizontal drainage capacity
| Pipe size | Max DFU @ 1/4" per ft | At 1/8" per ft (×0.8) |
|---|---|---|
| 1-1/4" | 1 | — |
| 1-1/2" | 1 | — |
| 2" | 8 | — |
| 3" | 35 | — |
| 4" | 216 | 172 |
| 5" | 428 | 342 |
| 6" | 720 | 576 |
| 8" | 2,640 | 2,112 |
| 10" | 4,680 | 3,744 |
| 12" | 8,200 | 6,560 |
How to use this chart
Total the DFU for every fixture the run serves — the DFU chart has the per-fixture values and the fixture unit calculator totals them — then pick the slope the site allows and read down that column. Steeper slope buys capacity; flatter slope is only available at all on larger pipe. Minimum slopes themselves are a separate rule, covered on the pipe slope chart. Stacks and horizontal fixture branches inside the building size from a different table — the drain pipe size chart.
IPC and UPC size the same run differently
A 6" building drain at 1/4" per foot carries 840 DFU under the IPC but 720 under the UPC — and the fixtures feeding it carry different DFU values in each code, so the totals diverge too. Work entirely inside one code family. Local amendments rewrite drainage tables often enough that the enforced edition and its amendments are the only final word; confirm with your jurisdiction.
Common questions
What size building drain do I need?
Total the drainage fixture units for everything the drain serves, pick your slope, and read down the column to the first size whose capacity covers the load. At 1/4" per foot under the IPC, a 4" building drain carries 216 DFU and a 6" carries 840. Any building drain serving a water closet is 3" minimum regardless of DFU math.
How does slope change drain capacity?
Flatter pipe moves less water. An 8" IPC building drain rated 1,920 DFU at 1/4" per foot drops to 1,600 at 1/8" and 1,400 at 1/16". The UPC handles it with a multiplier instead of columns: its horizontal capacities assume 1/4" per foot, and at 1/8" per foot you take 80% of the printed value.
What is the flattest slope allowed for a building sewer?
Size-dependent. Under the IPC, pipe smaller than 3" must fall at least 1/4" per foot, 3" through 6" at least 1/8", and only 8" and larger may run at 1/16". The UPC's default minimum is 1/4" per foot (2%), with 1% permitted for 4" and larger where the authority having jurisdiction approves it.
Is a building drain sized differently than a horizontal branch?
Yes — different tables. The building drain and sewer size from IPC Table 710.1(1), the slope matrix on this page. Horizontal fixture branches and stacks size from Table 710.1(2), which is on the drain pipe size chart. A 4" horizontal branch carries 160 DFU while a 4" building drain at 1/4" per foot carries 216.
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