Drainage is sized the same way as supply — fixture units summed against a table — with one important difference in the logic.
Typical DFU values
| Fixture | DFU, roughly |
|---|---|
| Basin | 1 |
| Bath | 2 |
| Shower | 2 |
| Kitchen sink | 2 |
| Dishwasher | 2 |
| Clothes washer | 2–3 |
| Water closet, 1.6 gpf | 3 |
| Floor drain | 2 |
| Full bathroom group | 5–6 |
Values differ between the IPC and the UPC, and a bathroom group is counted as a group rather than as the sum of its fixtures, because the fixtures in one bathroom are not used simultaneously.
Minimum sizes by fixture
Regardless of the unit count, each fixture has a minimum trap and drain size:
Basin — 1¼ inch.
Shower, kitchen sink, dishwasher — 1½ inch, and 2 inch for a shower under many codes.
Bath — 1½ inch, commonly run in 2.
Clothes washer standpipe — 2 inch.
Water closet — 3 inch.
Building drain — commonly 3 or 4 inch depending on the total load and the number of water closets.
Why oversizing is wrong
This is where drainage differs from supply.
A drain works partly by velocity: the flow has to move fast enough to carry solids along rather than letting them settle.
A pipe that is too large runs shallow and slow. Solids settle, accumulate and eventually block it.
A 4-inch branch serving one toilet is a common well-meant error, and it is worse than the 3-inch the code calls for.
Slope
Horizontal drains run at a specified fall, and it is not "as steep as possible".
¼ inch per foot for pipes up to 2½ inch.
⅛ inch per foot for 3 inch and larger, with ¼ inch also permitted.
Too little slope and the flow does not scour.
Too much slope, in a long run, and the liquid outruns the solids, which are left behind. Steep is not better either.
Fittings
Drainage fittings are directional. A long-sweep bend, a wye and a combination wye and eighth bend keep flow moving; a short-turn tee used on its back in a horizontal run creates a place for solids to stop.
A sanitary tee is for vertical-to-horizontal transitions, not horizontal-to-horizontal. Using the wrong fitting is one of the most common findings in amateur drainage work, and it does not show up until it blocks.
Cleanouts
Required at changes of direction, at intervals along long runs, at the base of stacks and where the building drain leaves the house.
They are what make a blockage a twenty-minute job. A house with no accessible cleanout gets its drains cleared through a removed toilet or from the roof.
Where the load adds up
The building drain carries everything, and it is sized on the total DFU with a limit on how many water closets a given size may serve.
Adding a bathroom adds to that total, which is why an addition sometimes requires the building drain to be upsized even though the existing one has never blocked.
The check worth making before a renovation
Count the DFU of the existing fixtures and of the proposed ones, and compare against the table for the existing building drain size.
That single calculation decides whether a new bathroom is a branch connection or a much larger project, and it is far cheaper to do on paper than to discover once the floor is up.
Which code applies
The IPC and the UPC assign slightly different unit values and different table limits, and jurisdictions adopt one or the other with local amendments.
The one adopted locally is the one that governs, and figures quoted online are frequently from the other. For anything being permitted, the local code's table is the only one that counts.