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Water Pipe Size Calculator
Size water supply piping from fixture unit counts, pressure and developed length.
Minimum service and main
1"Copper, Type L
- Total fixture units11 fixtures counted
- 21.7 WSFU
- Probable demandHunter's curve, flush-tank system
- 20.2 gpm
- Pressure available for friction18.78 psi per 100 ft over 180 ft
- 33.8 psi
- Largest branch10.7 gpm at 7.1 ft/s
- 3/4"
Compliant, but running near the 8 ft/s limit
| Nominal | Inside dia. | Velocity | Loss per 100 ft | Loss over run | Status |
|---|---|---|---|---|---|
| 1/2" | 0.545 in | 27.8 ft/s | 243.42 psi | 438.2 psi | Velocity too high |
| 3/4" | 0.785 in | 13.4 ft/s | 41.24 psi | 74.2 psi | Velocity too high |
| 1" — minimum | 1.025 in | 7.9 ft/s | 11.26 psi | 20.3 psi | Compliant |
| 1-1/4" | 1.265 in | 5.2 ft/s | 4.05 psi | 7.3 psi | Compliant |
| 1-1/2" | 1.505 in | 3.7 ft/s | 1.74 psi | 3.1 psi | Compliant |
| 2" | 1.985 in | 2.1 ft/s | 0.45 psi | 0.8 psi | Compliant |
| 2-1/2" | 2.465 in | 1.4 ft/s | 0.16 psi | 0.3 psi | Compliant |
| 3" | 2.945 in | 1.0 ft/s | 0.07 psi | 0.1 psi | Compliant |
| Item | Basis | psi |
|---|---|---|
| Static supply pressure | Measured with nothing running | 60.0 |
| Elevation rise | 12 ft × 0.433 psi/ft | −5.2 |
| Meter, backflow and treatment | As entered | −6.0 |
| Residual at the highest fixture | Required flowing pressure | −15.0 |
| Available for pipe friction | 56% of supply | 33.8 |
What this assumes
- Water supply fixture unit values follow the Uniform and International Plumbing Code fixture unit tables.
- Velocity is held at or below 8 ft/s for copper and 5 ft/s where erosion is a concern.
- Developed length includes an allowance for fittings expressed as equivalent length.
Important
Your jurisdiction adopts either the IPC or UPC, and the fixture unit values differ between them. Confirm which code applies before relying on a size for permitted work.
Understanding the result
Fixture units, not fixtures
Pipe is not sized by counting fixtures, because fixtures do not all run at once and do not all demand the same flow. Each fixture is assigned a water supply fixture unit value that reflects both its flow rate and how often it is used, and the total WSFU converts to a probable simultaneous demand in gallons per minute.
Pressure and length work against you
Available pressure is consumed by elevation (0.433 psi per foot of rise), by friction along the pipe, and by fittings. A long run to a second-floor bathroom can lose most of its static pressure, which is why the same fixture group can need a larger line in one house than another.
Why oversizing is not free
Oversized supply lines hold more water, so it takes longer for hot water to arrive at the fixture. That wastes water and energy every draw. Correct sizing — not maximum sizing — is what you are aiming for.
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