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Pipe & Drain Size Calculator — IPC Fixture Units
Three sizing problems, one tool: water-supply pipe from fixture units (WSFU), horizontal drain branches from drainage fixture units (DFU), and hydronic heating/cooling pipe from flow and allowed velocity.
System
Water supply fixture units
How fixture-unit sizing works
Plumbing codes size pipe by fixture units — probability-weighted demand values — rather than raw flow. For water supply, this calculator uses the IPC Appendix E Table E103.3(2) brackets for a system at 50–60 psi with up to 100 ft of developed length and a meter at least as large as the distribution pipe: 3/4" serves up to 8 WSFU, 1" up to 32, 1-1/4" up to 80, 1-1/2" up to 151, and 2" up to 365. The real appendix table is also meter-size dependent; this is the conservative single-column simplification.
For drainage, the DFU brackets follow IPC Table 710.1(2)'s horizontal-fixture-branch column: 1-1/2" up to 3 DFU, 2" up to 6, 2-1/2" up to 12, 3" up to 20, and 4" up to 160. One caveat the table alone won't tell you: any water closet needs a 3" minimum drain, and a 3" horizontal branch carries at most TWO water closets regardless of what the DFU math allows.
Hydronic mode sizes by velocity instead: 4 ft/s in occupied spaces, 6 in general areas, 8 in mechanical rooms — with copper additionally capped at 5 ft/s because hot-water erosion is a material limit that overrides the noise allowance.
Worked example
A branch serving 24 WSFU needs 1" supply pipe (the 1" bracket covers up to 32). On the drain side, 18 DFU on a horizontal branch needs 3" (the 3" bracket covers up to 20).
Frequently asked questions
What is a fixture unit?
A probability-weighted load value assigned to each plumbing fixture — WSFU for water supply demand, DFU for drainage load. Codes size pipe from fixture-unit totals because fixtures almost never all run at once, so raw flow would oversize everything.
Why might this differ from the full IPC table?
The full IPC Appendix E table varies by meter size, supply pressure band, and developed length. This calculator uses the conservative single-column simplification for 50–60 psi at up to 100 ft with a meter at least as large as the distribution pipe.
How many water closets can go on a 3 inch branch?
Two, maximum. Even where the DFU arithmetic would allow more, the IPC caps a 3" horizontal fixture branch at two water closets — and every water closet needs a 3" minimum drain.
Why is copper pipe limited to 5 feet per second?
Erosion. Copper's material velocity ceiling is about 5 ft/s for hot water at or below 140°F (and lower above that), independent of noise. Hydronic heating runs hot, so this calculator caps copper at 5 ft/s even in mechanical rooms.
Learn the code behind this calculator
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