Drain & Waste Sizing

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

5 articles

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.

72 calculations · browse all

Drainage Fixture Units — Sizing Drains and Why Bigger Is Not Better
A drain has to be big enough to carry the flow and small enough to keep it moving.
Slope and Fittings — Why Drains Block Where They Do
Drainage runs on gravity and geometry, and both are easy to get wrong in ways that only show up years later.
Traps — The Water Seal, and the Four Ways It Disappears
There is a small amount of water standing under every drain in the house, and it is the only thing between the room and the sewer.
The Line to the Street — Roots, Bellies and Camera Inspections
Everything in the house drains through one pipe, and it is the one nobody can see.
Draining a Basement Bathroom — Three Ways Up
Everything above the building drain drains itself. Everything below it needs a machine.