Pipe flow from velocity Calculator: measuring notes and method
Velocity is a required input
A diameter alone cannot predict flow. The average velocity must be known; the calculator does not derive it from supply pressure.
Check the operating condition
Use the actual internal diameter and a representative average velocity. A partly filled pipe requires a different cross-sectional area.
Formula
Flow = π × (internal diameter ÷ 2)² × average velocity. Convert cubic metres per second to the selected flow unit.
Worked examples in metric and imperial units
Illustrative values match Reset example and stay fixed when you edit the calculator. Both columns describe the same calculation; converted inputs are rounded.
Metric worked example
- Internal diameter
- 25 mm
- Average water velocity
- 1 m/s
Flow rate: 29.4524 L/min
Imperial worked example
- Internal diameter
- 0.9843 in
- Average water velocity
- 3.2808 ft/s
Flow rate: 7.7805 US gal/min
Follow the calculation
Lengths in this working are in metres.
- Cross-sectional area = π × (0.025 ÷ 2)² = 0.000491 m².
- Multiply area by 1 m/s for m³/s, then by 60,000 for L/min.
Reviewed 19 October 2026. How calculations are checked · Report a correction