Water velocity in pipes — why it matters

When a given flow has to pass through a pipe, the pipe's cross-section determines how fast the water has to move. A narrow pipe forces the water to flow faster at the same flow rate.

Watch the units: enter flow in l/s and diameter in mm, and convert to SI (÷1000) before inserting. Always use the internal diameter — not the nominal DN or outer dimension.

• 01Too high a velocity causes flow noise and can erode pipes and fittings over time.
• 02Too low a velocity can cause sedimentation and poor purging of air.
• 03In drinking water pipes, a maximum water velocity of approximately 1 m/s is recommended (DS 439/Viega — reduces biofilm and flow noise).

There is no universal speed limit. Copper is the most sensitive material: if water flows too fast — especially hot and with air bubbles or particles — the flow wears away the protective oxide layer on the inside, and erosion corrosion occurs (indentations and eventually perforation, typically in bends and after fittings). Stainless steel and plastic materials (PEX, PP) tolerate markedly higher velocities, because they are not dependent on a thin oxide layer in the same way. Therefore the limit is set lowest for circulating hot copper water.

The figures are indicative and vary between sources and water qualities. The copper figures come from the Copper Development Association / industry literature (cold ≈ 2.4 m/s, hot ≤ 60 °C ≈ 1.5 m/s, hot > 60 °C ≈ 0.6 m/s); the Danish recommendation to keep continuously circulating hot water below approx. 0.5 m/s recurs in Danish corrosion guidance. The ~1 m/s for domestic water is the industry practice DS 439/Viega works to. (Source: Copper Development Association / engineeringtoolbox; Danish EPA/danskbyggeskik corrosion guidance; DS 439:2024/Viega — indicative.)

Velocity determines not only noise and erosion, but also how much pressure is lost along the way. Pressure loss in a straight pipe is described by the Darcy-Weisbach equation — and it grows with the square of velocity, so doubling velocity quadruples pressure loss.

The friction factor λ depends on the Reynolds number Re = v · d / ν (ν = kinematic viscosity, for water ≈ 1.3 · 10⁻⁶ m²/s at 10 °C). At Re below approx. 2300 the flow is laminar and λ = 64 / Re; in practice water installations are almost always turbulent, and λ is then read from a Moody diagram or calculated iteratively with the Colebrook-White equation from Re and the relative roughness ε/d. Smooth materials (copper, plastic) have low roughness and therefore lower λ than old, corroded galvanised steel.