Drain & Waste Sizing

DFU to drain size, and the slope every horizontal run needs.

What size building drain for a house with 4 half bathrooms?

4 half bathrooms — building drain

3″

25 drainage fixture units at a ¼″ per foot fall — a 3″ drain carries 42

Building drainat ¼″ per foot of fall	3″
Total drainage fixture units4 × 4 for the bathrooms + 9 for kitchen and laundry	25DFU
Capacity of a 3″ drain17 DFU of headroom left	42DFU
Branch serving one bathroom4 DFU — toilet and basin only — no bath or shower	2″
Water supply fixture units	17WSFU
Supply mainHunter curve at 46–60 psi street pressure	¾″

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 4 half bathrooms plus a kitchen and laundry comes to 25 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. 17 WSFU calls for a **¾″ 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.

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