Pipe Velocity Check
Sediment under 0.6 m/s, water-hammer over 3.0 m/s. The band check + inverse-diameter helper, in one quick page.
Inputs
Velocity
0 ft/s
1.556 m/sSafe operating velocity
SedimentSafeHighHammer risk
012345 m/s
Inverse: pick a diameter for a target velocity
Required ID (exact)
0 in
46.9 mmNext-up Sch-40 nominal
2"NPS
ID ≈ 2.067 inHow this works
Velocity: v = Q / A = 4Q / (π · d²) Inverse (diameter for target v): d = √(4Q / (π · vtarget))
The math is simple — what matters is the design rule. Plumbing codes typically design cold water around 5-7 ft/s. Hot water gets dropped 1-2 ft/s lower because erosion-corrosion of copper accelerates with temperature × velocity. Industrial process lines may push higher when the fluid and pipe lining permit.
Water hammer (a transient pressure spike from sudden valve closure) scales with velocity squared. Above 3 m/s the energy in the moving column is high enough to damage copper joints, push past solenoid-valve closure ratings, and produce the telltale "knocking" that annoys building tenants.
| ft/s | m/s | Reason | |
|---|---|---|---|
| Cold water (domestic) | 5-8 | 1.5-2.4 | Standard ASPE band |
| Hot water (≥ 50°C) | 3-5 | 0.9-1.5 | Erosion-corrosion of copper |
| Chilled water (HVAC) | 4-8 | 1.2-2.4 | Balance friction vs noise |
| Steam (low-pressure) | 25-50 | 7.6-15 | Vapor — different physics |
| Pump suction | 2-4 | 0.6-1.2 | Avoid NPSH pressure drop |
| Pump discharge | 5-10 | 1.5-3 | Above NPSH limit |
Common questions
Why are these the band thresholds?
They come from the ASPE Plumbing Engineering Design Handbook (vol. 4, ch. 4) and standard plumbing codes. Below 0.6 m/s, sediment settles in the pipe and biofilm grows; above 2.4 m/s noise becomes audible and erosion accelerates; above 3.0 m/s a sudden valve closure can produce damaging water-hammer pressure spikes.
Is target velocity really the right design target?
For cold water in plumbing-scale pipes, yes — sizing for 5-7 ft/s is standard practice. For chilled-water and hot-water hydronic loops, energy cost may favor lower velocities; for industrial process lines, vendors may push higher with anti-erosion linings. Use this as a starting point, not a rule.
How does the inverse-diameter calc handle non-standard pipe?
It computes an exact diameter for the target velocity, then snaps to the next-up Schedule 40 nominal size. If your pipe is metric (DN) or non-Schedule-40, use the exact diameter and ignore the nominal recommendation.