Pipe Flow Rate Calculator
Estimate flow rate through a pipe from its diameter and velocity, plus friction head loss for water.
About the Pipe Flow Rate Calculator
Enter the pipe's internal diameter, the flow velocity, the length and the Hazen–Williams roughness. The calculator gives the flow rate in L/s, m³/h and US gal/min, and the friction head loss for water.
Flow rate is the pipe's cross-sectional area times average velocity. Water supply pipes are usually sized for 1–2 m/s to limit noise and pressure loss. Head loss uses the empirical Hazen–Williams formula, valid for water at normal temperatures in turbulent flow.
The formula
Q = π D² ÷ 4 × v; hf = 10.67 L Q^1.852 ÷ (C^1.852 D^4.87)
Worked example
A 50 mm pipe at 1.5 m/s, 100 m long, C = 140
- Area: π × 0.05² ÷ 4 = 0.00196 m²
- Flow: 2.945 L/s = 10.6 m³/h
- Head loss about 5.05 m (49.5 kPa)
Questions people ask
Why does diameter matter so much?
Area grows with D², and friction loss falls roughly with D^4.87.
What about other fluids?
Use the Darcy–Weisbach equation with the fluid's viscosity.