Water Flow Rate in Pipe Calculator

Volumetric flow rate Q = A * v, where A is the cross-sectional area of the pipe (pi * r^2) and v is the average flow velocity. For a circular pipe: Q = pi * (D/2)^2 * v, where D is the internal diameter. Units: if D is in metres and v is in m/s, Q is in m3/s. Multiply by 1,000 to convert to litres per second.

Reynolds number (Re) = rho * v * D / mu, where rho is fluid density (1,000 kg/m3 for water at 20 C), v is velocity (m/s), D is pipe diameter (m), and mu is dynamic viscosity (0.001 Pa*s for water at 20 C). Re below 2,300 indicates laminar flow; Re above 4,000 indicates turbulent flow. Flow regime affects friction losses and pump sizing.

Laminar flow (Re below 2,300) is smooth and orderly, with water layers sliding past each other. Turbulent flow (Re above 4,000) involves chaotic mixing, higher friction losses, and greater energy requirements. Most practical water supply and marine systems operate in the turbulent regime. The transition zone (2,300 to 4,000) is unstable.

For domestic water supply pipes, typical velocities are 0.5 to 2 m/s (1.6 to 6.6 ft/s). Velocities above 3 m/s in steel pipes can cause erosion and noise. For fire suppression systems, 3 to 6 m/s is common. AWWA and ASPE guidelines recommend keeping velocities below 3 m/s in most applications to minimise pressure losses and water hammer.

1 m3/s = 1,000 litres/second = 264.17 US gallons/second = 15,850 US gallons/minute (gpm). For marine bilge pumps and plumbing, gph (gallons per hour) is commonly used: 1 m3/hr = 264.17 gph. Always check the units when sizing pumps from manufacturer data sheets.