Size a domestic water pipe from the design flow or fixture units. The calculator checks the velocity, the friction loss and, if you want, the whole pressure budget from the supply to the highest fixture, and recommends the smallest size that works.
The IPC does not set a maximum velocity, but high velocity causes noise, water hammer and erosion of copper. Common limits are about 2.4 m/s (8 ft/s) for cold water, 1.5 m/s (5 ft/s) for hot water up to 60 °C, and 0.6–0.9 m/s (2–3 ft/s) for water routinely above 60 °C. Branches near bedrooms and meeting rooms are often kept to about 1.2 m/s.
| Fixture | Minimum flow pressure |
|---|---|
| Faucets, lavatories, sinks | 8 psi (55 kPa) |
| Flush tank water closet, shower or tub mixing valve | 20 psi (138 kPa) |
| Urinal flush valve | 25 psi (172 kPa) |
| Siphonic flushometer water closet | 35 psi (241 kPa) |
| Blowout flushometer water closet | 45 psi (310 kPa) |
If the budget is negative, no pipe size can fix it: a booster pump set is needed. Get the WSFU from the Fixture Unit Calculator, check any run in detail with the Water Pipe Pressure Drop Calculator, and size a booster with the Pump Head Calculator.
Convert the fixture units to a design flow with Hunter's curve, work out the allowable friction loss from the pressure budget, then choose the smallest pipe whose friction loss and velocity are within limits at that flow.
In Type L copper, 1 inch runs at about 7.8 ft/s, right at the usual 8 ft/s limit, with about 11 psi of friction per 100 ft. 1¼ inch (about 5.1 ft/s and 4 psi per 100 ft) is the more common choice. Use the calculator with your pressure budget for the exact size.
IPC Table 604.3 requires 35 psi for a siphonic flushometer valve water closet and 45 psi for a blowout type, much more than the 20 psi of a flush tank toilet.
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