Pipe Size Calculator
Enter flow and your velocity ceiling (supply 2.4 m/s, hot 1.5, pump suction 1.2) to get the minimum bore and the next standard size up.
Last updated: 2026-09-28
How the calculation works
- Inverts the velocity relation: the smallest bore that keeps the flow at or below the velocity ceiling.
- Snaps up to the next standard trade size from a 8–150 mm internal-diameter series.
- Reports the actual velocity at the picked size so you can see the margin.
Formula
D = √(4Q / (π·V_limit))
| Symbol | Meaning | Unit |
|---|---|---|
D | Minimum internal diameter | mm |
V_limit | Design velocity ceiling | m/s |
Worked example
Interpreting the result
Sizing to the velocity ceiling is the fast first pass; real designs also check the pressure budget (friction over the whole run against available pressure) and fixture-unit demand for branch sizing. When in doubt, go one size up: the pipe cost difference is trivial next to the noise, hammer and friction a tight bore adds for the life of the building.
Assumptions
- Water, full-bore steady flow.
- Standard trade internal diameters (8–150 mm series).
Limitations
- Fixture-unit demand sizing (probable simultaneous flow) is a separate method — this sizes for a known peak flow.
- Gas and compressed-air lines follow different velocity conventions.
Frequently asked questions
What size pipe for 30 L/min?
At a 2.4 m/s supply limit: 20 mm internal. For hot water or long runs, check the pressure loss — 25 mm may still pay for itself.
Why limit velocity in pipes?
Noise, water hammer and erosion-corrosion all scale with velocity — and friction loss scales with V², quietly eating the pressure budget on every metre.