Pipe Flow Calculator
Enter flow and pipe size to get velocity and the verdict: good supply range, noisy, or hammer-and-erosion territory.
Last updated: 2026-09-28
How the calculation works
- Velocity divides the flow by the pipe's internal area — internal diameter, not nominal size.
- The verdict bands come from supply design practice: 0.6–2.4 m/s is the cold-water comfort zone.
- Imperial equivalents (ft/s, gpm) shown for US-size cross-checking.
Formula
V = Q / A A = π·D²/4
| Symbol | Meaning | Unit |
|---|---|---|
V | Flow velocity | m/s |
Q | Volumetric flow | L/min |
A | Internal cross-section | mm² |
Worked example
30 L/min in a 20 mm ID pipe: A = 314 mm², V = 0.5/0.000314 = 1.59 m/s — mid-range, quiet and safe. The same flow in 15 mm ID: 2.83 m/s — noisy and hammer-prone.
Interpreting the result
Velocity is the first check in pipe sizing: friction loss scales with V², so doubling velocity quadruples pressure loss per metre. Supply lines live at 0.6–2.4 m/s; hot water stays near 1.5 m/s to limit erosion-corrosion in copper. Drainage lines run slower by design; suction lines of pumps must stay under ~1.2 m/s or cavitation follows.
Assumptions
- Water at ambient temperature; full-bore flow.
- Internal diameter entered directly (use the pipe's ID table for the actual bore).
Limitations
- Does not compute pressure loss — use the pipe pressure drop calculator for the friction figure.
- Partially-full drainage flow follows different hydraulics (open-channel).
Frequently asked questions
What velocity should water flow in a pipe?
Cold supply: 0.6–2.4 m/s (2–8 ft/s). Hot water: ≤1.5 m/s. Above ~3 m/s you get noise, hammer and erosion — especially in copper.
How do I convert GPM to velocity?
Velocity = GPM × 3.785 ÷ 60 ÷ (π/4 × ID²) with ID in metres — or just enter gpm here and read the answer.