Water Pipe Size Calculator with Fixture Units
Count your fixtures, convert water supply fixture units to peak demand in gpm and L/s with the IPC Hunter’s curve tables, and find the smallest copper, PPR, PEX or steel pipe that meets your velocity and friction limits. Modelling the plumbing too? Browse our MEP Revit families.
WSFU from IPC Table E103.3(2)Hunter’s curve, tank or valveCopper, PPR, PEX, steelHazenโWilliams frictionPressure left at the fixture
Building and fixtures
Private: houses and apartments, used by one family. Public: offices, schools, hotels, restaurants and other shared toilets.
Number of fixtures served by the pipe you are sizing. Values are water supply fixture units (wsfu) from IPC 2021 Table E103.3(2).
2.2 wsfu each
6 wsfu each
2 wsfu each
10 wsfu each
5 wsfu each
3 wsfu each
0.7 wsfu each (cold 0.5, hot 0.5)
1.4 wsfu each (cold 1, hot 1)
1.4 wsfu each (cold 1, hot 1)
1.4 wsfu each (cold 1, hot 1)
2 wsfu each (cold 1.5, hot 1.5)
1.4 wsfu each (cold 1, hot 1)
4 wsfu each (cold 3, hot 3)
1.4 wsfu each (cold 0, hot 1.4)
1.4 wsfu each (cold 1, hot 1)
3 wsfu each (cold 2.25, hot 2.25)
3 wsfu each (cold 2.25, hot 2.25)
0.25 wsfu each
3.6 wsfu each (cold 2.7, hot 1.5)
8 wsfu each (cold 6, hot 3)
Continuous demand: 5 gpm (0.32 L/s) each, added to the peak flow.
Irrigation, cooling make-up and similar loads.
Demand
Auto uses the flush-valve curve when any flush valve is connected, except for hot-only pipes.
Pipe and limits
150 for copper and plastics, 120 for galvanised steel.
2.4 m/s (8 ft/s) for cold water in copper; use 1.5 m/s (5 ft/s) for hot water.
Design choice. 3โ5 psi per 100 ft (about 7โ11 m per 100 m) is a common target.
Pressure at the fixture (optional)
Enter the minimum pressure available at the start of the pipe to check what is left at the most remote fixture. Leave it empty to skip the check.
Added to the length for elbows, tees and valves.
IPC Table 604.3: 8 psi (55 kPa) for most fixtures, 20 psi for tank WCs, 25โ45 psi for flush valves.
Recommended pipe size
Enter your fixtures to see the result.
Preliminary sizing only. This follows the IPC Appendix E fixture-unit method with HazenโWilliams friction for new, clean pipe. Hunter's curves can overestimate demand in buildings with modern low-flow fixtures. Follow your local plumbing code (IPC, UPC, the Egyptian code for sanitary installations or your local authority rules), check the minimum branch sizes it sets and confirm the design with a licensed engineer.
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Request a projectSize a water pipe in three steps
Count the fixtures
Choose private (dwelling) or public use and enter how many WCs, basins, sinks, showers, baths and appliances the pipe serves, plus any hose bibbs.
Get the peak demand
The total fixture units are converted to a peak flow with the flush-tank or flush-valve curve of IPC Table E103.3(3), with continuous loads added on top.
Pick the pipe
Select the material and limits. The calculator lists every size with velocity and head loss and recommends the smallest that passes, and can check the pressure left at the most remote fixture.
From fixture units to pipe size
Q is in mยณ/s, D is the inside diameter in m and S is the head loss in metres per metre. C is 150 for copper, PPR and PEX and 120 for galvanised steel. Hunter’s curves are probability curves: they convert the sum of fixture units into the flow you can expect at peak, which is far less than every fixture running at once.
Worked example: a house with 2 tank WCs (2 ร 2.2), 2 basins (2 ร 0.7) and six 1.4-unit fixtures (a kitchen sink, 2 showers, a bath, a washing machine and a dishwasher) totals 4.4 + 1.4 + 8.4 = 14.2 WSFU. The flush-tank curve gives 17.0 gpm at 14 and 17.5 gpm at 15, so 17.1 gpm (1.08 L/s). In 1 in type L copper that flows at 2.03 m/s with 16.6 m of head loss per 100 m (7.2 psi per 100 ft); 1 1/4 in drops this to 1.33 m/s and 6.0 m per 100 m.
Cold water in copper is usually kept to 8 ft/s (2.4 m/s) and hot water to about 5 ft/s (1.5 m/s) to limit noise and erosion-corrosion.
WSFU to gpm and L/s (IPC Table E103.3(3))
| Load (WSFU) | Flush tanks (gpm) | Flush tanks (L/s) | Flush valves (gpm) | Flush valves (L/s) |
|---|---|---|---|---|
| 5 | 9.4 | 0.59 | 15.0 | 0.95 |
| 10 | 14.6 | 0.92 | 27.0 | 1.70 |
| 20 | 19.6 | 1.24 | 35.0 | 2.21 |
| 50 | 29.1 | 1.84 | 50.0 | 3.15 |
| 100 | 43.5 | 2.74 | 67.5 | 4.26 |
| 200 | 65.0 | 4.10 | 90.0 | 5.68 |
| 500 | 124.0 | 7.82 | 143.0 | 9.02 |
| 1,000 | 208.0 | 13.12 | 208.0 | 13.12 |
| Fixture (IPC Table E103.3(2)) | Private (WSFU) | Public (WSFU) |
|---|---|---|
| Water closet, flush tank | 2.2 | 5 |
| Water closet, flush valve | 6 | 10 |
| Lavatory | 0.7 | 2 |
| Kitchen sink | 1.4 | 4 (hotel, restaurant) |
| Shower head | 1.4 | 4 |
| Bathtub | 1.4 | 4 |
| Washing machine (8 lb) | 1.4 | 3 |
| Dishwasher | 1.4 | โ |
| Urinal, 3/4 in flush valve | โ | 5 |
Total loads shown. Values between rows are interpolated linearly. The two curves meet at 1,000 WSFU, and the table ends at 5,000 WSFU.
Water pipe sizing questions
Add up the water supply fixture units of every fixture the pipe serves, then read the peak demand from IPC Table E103.3(3). Use the flush-valve column if the system is mainly flush valves and the flush-tank column otherwise, and interpolate between rows. For example, 20 WSFU is 19.6 gpm with flush tanks and 35 gpm with flush valves.
A water supply fixture unit (WSFU) is a load factor that reflects how much water a fixture uses, how fast and how often. A private flush-tank WC is 2.2 WSFU and a private basin is 0.7, while a public flush-valve WC is 10. Because not all fixtures run at once, you add the units and convert the total to a peak flow instead of adding flow rates.
A typical house with two bathrooms totals about 14 WSFU, roughly 17 gpm (1.1 L/s) at peak. In copper that usually needs 1 in to keep velocity under 8 ft/s, or 1 1/4 in if you also limit friction to about 4 psi per 100 ft or have low street pressure. Long runs and upper floors need a pressure check.
Use the inside diameter, not the outside diameter. A PN20 PPR pipe has thick walls (SDR 6), so a 32 mm pipe has only 21.2 mm inside and a 40 mm pipe 26.6 mm. Work out the peak flow from the fixture units, then pick the smallest PPR size that keeps velocity and friction within your limits; select PPR PN20 in the calculator to see every size.
The Copper Development Association recommends no more than 8 ft/s (about 2.4 m/s) for cold water and 5 ft/s (1.5 m/s) for hot water up to 140 ยฐF (60 ยฐC), dropping to 2โ3 ft/s above that. Higher speeds cause noise and erosion-corrosion, especially at elbows and tees.
Use the flush-valve curve when the building is mainly served by flushometer valves (offices, schools, public toilets) and the flush-tank curve for homes and buildings with tank WCs. Flush valves draw a high flow for a short time, so the same load gives a higher peak demand below 1,000 WSFU. Hot-water pipes use the flush-tank curve.
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