Why water velocity matters
The speed of water in a pipe drives noise, pressure loss, water hammer and erosion. Too fast and pipes whistle, fittings wear and valves slam; too slow and pipes are oversized, more expensive and, in hot water lines, slow to deliver hot water.
The formulas
- Area = π × d² ÷ 4, using the inner diameter
- Velocity v = Q ÷ A
- Reynolds number Re = v × d ÷ ν, with ν ≈ 1.0 × 10⁻⁶ m²/s for water at 68 °F (20 °C)
- Diameter for a target velocity d = √(4Q ÷ (π × v))
Worked example
A shower and a kitchen faucet together draw 10 gpm through a pipe with an inner diameter of 0.75 in:
- Q = 10 gpm = 0.000631 m³/s.
- A = π × 0.01905² ÷ 4 = 0.000285 m².
- v = 0.000631 ÷ 0.000285 ≈ 2.21 m/s = 7.26 ft/s.
- Re ≈ 42,000: fully turbulent flow.
- To bring the velocity down to 5 ft/s, the inner diameter would need to be about 0.90 in, roughly a 1 in nominal pipe.
Recommended velocities
| Application | Typical maximum |
|---|---|
| Cold water supply, copper | 8 ft/s (2.4 m/s) |
| Hot water supply, copper | 4–5 ft/s (1.2–1.5 m/s) |
| Hot water recirculation | 2–3 ft/s (0.6–0.9 m/s) |
| Pump suction lines | 2–4 ft/s (0.6–1.2 m/s) |
| Hydronic heating | 2–4 ft/s (0.6–1.2 m/s) |
| Irrigation (PVC) | 5 ft/s (1.5 m/s) |
Copper Development Association guidance and many plumbing codes use these figures to limit erosion corrosion and noise.
Inner diameters of common pipes
| Nominal size | Type L copper | PEX (SDR 9) | Schedule 40 PVC |
|---|---|---|---|
| 1/2 in | 0.545 in | 0.475 in | 0.622 in |
| 3/4 in | 0.785 in | 0.671 in | 0.824 in |
| 1 in | 1.025 in | 0.862 in | 1.049 in |
Always use the inner diameter: nominal size alone can be off by 20% or more.
Laminar or turbulent?
Below Re ≈ 2,300 flow is smooth and layered; above about 4,000 it is turbulent and mixes strongly. Domestic water flow is almost always turbulent, so friction loss formulas such as Hazen-Williams or Darcy-Weisbach with a turbulent friction factor apply.
Next step
Velocity is only half the story. Long runs also need a pressure loss check to be sure enough pressure remains at the farthest fixture. Combine this result with the pump power calculator when you size a booster or well pump.