Code Applied by the app plumb.drain.dfu-capacity
Building drain and sewer capacity in drainage fixture units
How many drainage fixture units a building drain or sewer may carry by size and slope; at 1/8 inch per foot a 3 inch sewer carries 36 and the course's 4 inch carries 180.
| When | The app uses |
|---|---|
| 3 in building drain or sewer, at 1/8 in per foot | 36 DFU |
| 3 in building drain or sewer, at 1/4 in per foot | 42 DFU |
| 3 in building drain or sewer, at 1/2 in per foot | 50 DFU |
| 4 in building drain or sewer, at 1/8 in per foot | 180 DFU |
| 4 in building drain or sewer, at 1/4 in per foot | 216 DFU |
| 4 in building drain or sewer, at 1/2 in per foot | 250 DFU |
- Source
- IPC · 710.1, Table 710.1(1) · read the public text
- Editions checked
- 2018 · 2021
- Used by
- Courses
- Amendments
No state or local amendment to this rule is on file. Not on file means the model code as written — check your jurisdiction.
A model code the authority having jurisdiction adopts.
The code rates every fixture in drainage fixture units, a measure of how hard it loads the drain (a flush valve water closet sends its whole flush in seconds, a lavatory drains a trickle), and sizes the pipe under it from the total. The building drain and the sewer carry everything, so their size is read from the building's whole load at the slope they are laid to. The course's set adds the load to 47 on P-501, well inside the 180 its 4 inch sewer carries at 1/8 inch per foot.
On a bid the check is quick and worth doing: a sewer too small for the load is a bigger pipe and often a deeper trench.
What the app does with it
Nothing yet: the app does not apply this rule. It is on a course card (the plumbing course, Chapter 9, "What the rows mean"). Bid Check has a manual row, Fixture units checked against the building drain size, that you tick when you have read the engineer's total against this table. The app counts no drainage fixture units; the water side's fixture units are a different table (plumb.wsfu.fixtures).
Verify against your edition
Only the 3 and 4 inch rows the course uses are here; the table covers every drain size. The per-fixture drainage units are IPC Table 709.1, which the sample sheet cites for its DFU column. The six figures matched the 2021 text of Table 710.1(1), whose 1/16 inch column is blank at 3 and 4 inch, and whose footnote keeps a building drain that serves a water closet at 3 inch or larger. The Uniform Plumbing Code sizes drains by its own tables in Chapter 7, with different figures. Settled 2026-09-27 by Claude on the owner's delegation, from the dossier's research and the model code text; not checked against a printed book or a local amendment.
Cited by section, not reprinted: the code text belongs to its publisher. Company practice is not a rule here — it lives with your pricing.