WATER SUPPLY PIPE
Water supply fixture unit (WSFU) chart — IPC Table E103.3(2)
Private-occupancy fixture unit values — the column that applies to a house. The public column in the same code table runs much higher, and using it on a residence oversizes the whole system: a public 15 lb washing machine is 4.0 WSFU against 1.4 for the private 8 lb machine.
| Fixture | WSFU (private) |
|---|---|
| Bathroom group, flush-tank water closet | 3.6 |
| Half bath (water closet 2.2 + lavatory 0.7) | 2.9 |
| Water closet, flush tank | 2.2 |
| Bidet | 2.0 |
| Bathtub | 1.4 |
| Shower head | 1.4 |
| Kitchen sink | 1.4 |
| Dishwashing machine | 1.4 |
| Clothes washer, 8 lb | 1.4 |
| Laundry tray | 1.4 |
| Lavatory | 0.7 |
| Hose bib | not a fixture unit — add 5 gpm continuous |
IPC 2021 Table E103.3(2), private occupancy. Hose bib handling per IPC Appendix E.
Fixture units are only step one — demand comes from Table E103.3(3) and pipe size from Table E104.1, and both depend on pressure and developed length. Run the full sizing above.
About this calculator
This water supply pipe sizing calculator sums Water Supply Fixture Units from IPC Table E103.3(2) using the PRIVATE occupancy values, converts that load to peak demand through Table E103.3(3), and reads the minimum distribution pipe size out of Table E104.1. Two things most WSFU calculators leave out change the answer: hose bibs are not fixture units at all — IPC Appendix E adds them as continuous demand on top of the fixture-unit total — and pipe size depends on available pressure and the developed length of the run, not on fixture units alone. The same ¾-inch distribution pipe carries 9.5 WSFU on a 40-foot run and 1.5 WSFU at 500 feet in the lowest pressure band. ESTIMATE ONLY — verify with a licensed plumber and local plumbing code before installation.
How to use this calculator
Enter the count of each fixture the supply serves. The calculator uses IPC Table E103.3(2) private-occupancy values: a full bathroom group is 3.6 WSFU, a half bath is 2.9 (2.2 water closet + 0.7 lavatory), and kitchen sink, dishwasher and clothes washer are 1.4 each.
Hose bibs are counted differently and this matters. They are not in the fixture-unit table at all — IPC Appendix E adds them as continuous demand, 5 gpm each, on top of the fixture-unit total, because a hose runs wide open for an hour rather than in the short bursts the probability curve assumes.
Set your static pressure band and the maximum developed length — the run from the meter to the most remote fixture, times about 1.2 to allow for fittings. Pipe size falls out of Table E104.1 for that pressure and length, not from fixture units alone.
Worked example
Typical 2-bath house: 2 full baths, kitchen sink, dishwasher, clothes washer, 2 hose bibs. 50-60 psi, 100 ft developed length.
WSFU: (2 × 3.6) + 1.4 + 1.4 + 1.4 = 11.4.
Fixture demand from Table E103.3(3): about 15.6 GPM. Hose bibs add 10 GPM continuous, so peak demand is roughly 25.6 GPM.
Table E104.1 at 50-60 psi and 100 ft: a ¾-inch distribution pipe carries 8.5 WSFU, not enough. The next row carries 32. Result: ¾-inch meter and service, 1-inch distribution pipe.
Same house on a long rural run — 30-39 psi and 400 ft: that 1-inch pipe now carries only 5.5 WSFU, and the ¾-inch row only 2. You need 1¼-inch distribution pipe for the identical fixture count. Distance and pressure, not fixtures, moved the answer.
One-bath cabin — 1 bath, kitchen sink, clothes washer, no hose bibs: 6.4 WSFU, about 11.1 GPM, and ¾-inch distribution pipe covers it at 50-60 psi over 100 ft.
Common mistakes & waste factors
Counting hose bibs as fixture units. They are not in Table E103.3(2). IPC Appendix E adds them as flat continuous demand — a hose runs wide open, which is exactly what the fixture-unit probability curve is not designed for.
Using public fixture values on a house. Table E103.3(2) lists both. A public 15-lb washing machine is 4.0 WSFU; the private 8-lb machine is 1.4. Picking the wrong row nearly triples that fixture.
Sizing on fixture units alone. Table E104.1 is keyed by pressure band and developed length as well. A ¾-inch distribution pipe carries 9.5 WSFU at 40 ft and 1.5 WSFU at 500 ft in the lowest pressure band.
Using nominal PEX size as if it were copper. PEX has a smaller inside diameter at the same nominal size — bump one size when substituting.
Confusing static and dynamic pressure. Static is the no-flow reading; sizing is driven by what is left once the run, the meter and the fittings have taken their share.
Rules of thumb
WSFU, IPC Table E103.3(2) private column: bathroom group with flush tank 3.6, water closet flush tank 2.2, lavatory 0.7, bathtub 1.4, shower head 1.4, kitchen sink 1.4, dishwasher 1.4, washing machine (8 lb) 1.4, laundry tray 1.4, bidet 2.0.
Hose bibs: not fixture units. Add 5 gpm each as continuous demand.
Demand comes from Table E103.3(3), not a square-root approximation. At 30 WSFU the code says 23.3 GPM; a common fitted curve gives about 14.9, roughly 36% low.
Pipe size comes from Table E104.1 by pressure band and developed length. Longer run or lower pressure means a bigger pipe for the same fixtures.
PEX: bump one nominal size from the copper answer.
IPC Section 604.1 sets a ¾-inch minimum water service pipe.
Common questions
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