What size building drain for a house with 8 half bathrooms?
| Building drainat ¼″ per foot of fall | 3″ |
|---|---|
| Total drainage fixture units8 × 4 for the bathrooms + 9 for kitchen and laundry | 41DFU |
| Capacity of a 3″ drain1 DFU of headroom left | 42DFU |
| Branch serving one bathroom4 DFU — toilet and basin only — no bath or shower | 2″ |
| Water supply fixture units | 27WSFU |
| Supply mainHunter curve at 46–60 psi street pressure | 1″ |
Notes
- **Fixture units are not fixtures.** A DFU is a unit of *probable simultaneous flow*, not a count of drains. Six fixtures do not all discharge at once, and the tables are built on that probability — which is why 8 half bathrooms plus a kitchen and laundry comes to 41 DFU rather than a simple headcount.
- **Slope changes the answer more than most people expect.** The same 3″ pipe carries — DFU at ⅛″ per foot and 50 at ½″. Too little fall and solids strand; too much and the water outruns them, which strands solids for the opposite reason. ¼″ per foot is the usual target.
- **A 3″ drain is capped by more than DFU.** Most codes also limit how many water closets a 3″ line may serve regardless of the arithmetic, and at ⅛″ fall a 3″ building drain is not permitted at all. Check the water-closet limit as well as the number above.
- The supply side answers a different question. 27 WSFU calls for a **1″ main** at typical street pressure — but length of run, elevation gain and meter size all pull that around, and a long run to an upstairs bathroom is the case where the table alone gets it wrong.
- Figures here are the IPC. Jurisdictions on the UPC use different fixture-unit values for several fixtures, and the local inspector is the one who decides. Use this to size a plan and to check a quote, not to argue with an inspection.
8 half bathrooms — the drain size
3″, carrying 41 drainage fixture units at 1/4 inch per foot of fall.
That is 8 bathrooms plus a kitchen and a laundry. The supply side needs a 1″ main.
Fixture units are not fixtures
A DFU measures probable simultaneous flow, not a count of drains. Six fixtures never discharge at once, and the code tables are built on that probability.
It is why the honest answer is 41 DFU and not a headcount of everything with a plughole.
Slope changes the answer
The same pipe carries a different load at every fall. Too little and solids strand. Too much and the water outruns them — which strands solids for the opposite reason.
¼ inch per foot is the usual target, and the one most inspectors expect to see.
The 3-inch limit nobody mentions
Most codes cap how many water closets a 3″ line may serve regardless of the DFU arithmetic, and at ⅛″ of fall a 3″ building drain is not permitted at all.
Check that limit as well as the number above.
The supply side is a different question
27 WSFU calls for a 1″ main at typical street pressure. Length of run, elevation and meter size all pull that around — and a long run to an upstairs bathroom is exactly where the table alone gets it wrong.
Figures here follow the IPC. UPC jurisdictions use different values for several fixtures, and the inspector decides.
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