Code Applied by the app plumb.water.fixture-supply-min
Minimum fixture supply pipe sizes
The smallest pipe that may supply each fixture, 3/8 inch for a lavatory or a tank water closet up to 1 inch for a flush-valve water closet.
| When | The app uses |
|---|---|
| Bathtub | 1/2 in |
| Bidet | 3/8 in |
| Combination sink and tray | 1/2 in |
| Dishwasher, domestic | 1/2 in |
| Drinking fountain | 3/8 in |
| Hose bibb | 1/2 in |
| Kitchen sink | 1/2 in |
| Laundry, 1, 2 or 3 compartments | 1/2 in |
| Lavatory | 3/8 in |
| Shower, single head | 1/2 in |
| Sink, flushing rim | 3/4 in |
| Sink, service | 1/2 in |
| Urinal, flush tank | 1/2 in |
| Urinal, flush valve | 3/4 in |
| Wall hydrant | 1/2 in |
| Water closet, flush tank | 3/8 in |
| Water closet, flush valve | 1 in |
| Water closet, flushometer tank | 3/8 in |
| Water closet, one piece | 1/2 in |
- Source
- IPC · 604.5, Table 604.5 · read the public text
- Editions checked
- 2018 · 2021
- Used by
- Water Sizing
- 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.
Whatever the fixture-unit arithmetic says, a fixture's supply pipe has a floor. A flush-valve water closet needs a 1-inch supply because the valve draws hard and briefly; a lavatory's 3/8-inch stop and riser are enough for its faucet. The floor is on the fixture supply, the pipe from the branch to the fixture, which the code ends within 30 inches of its point of connection to the fixture.
What the app does with it
When a fixture is attached to a water run (rung 3 of the water-sizing ladder), the run's size is compared with the floor for that fixture; a branch smaller than its fixture allows is flagged ⚠ in the Water Sizing schedule and named in Bid Check's Fixture supply minimums row ("WC flush valve on 3/4 in; needs 1 in", rung 6). The size suggestion never offers a size under a served fixture's floor.
What it does not do
It does not size the fixture's own supply riser or stop; those are counted with the fixture. It does not measure the 30 inches from the end of the fixture supply to the fixture.
It does not apply the table's footnote for a parallel (manifold) system: there a line from the manifold to one fixture may be one nominal size smaller than the table, when the line's developed length is 50 ft or less and the pressure at the meter is 35 psi or more (the table's footnote a). The Water Sizing schedule and Bid Check still flag such a line as under its fixture's minimum; an estimator who has read the footnote on a home-run PEX job leaves the size as drawn.
Verify against your edition
Every value matched the 2021 text of Table 604.5 as state adoptions print it, and the 2018 edition carries the same table under the same number. (This rule, water-model.js and the water-sizing plan cited it as Table 604.4 until 2026-09-27; 604.4 is the maximum flow rate table.) The footnote's two limits, 50 ft of developed length and 35 psi at the meter, were read 2026-09-27 in the IPC 2021 as Colorado adopts it (UpCodes); the dossier had read the same, and a 60 ft figure remembered against it was wrong. 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.