Calculate water flow rate from pipe diameter and flow velocity.
How to Use the Water Flow Calculator
Enter the "Pipe Diameter (mm)" — use the internal (bore) diameter of the pipe.
Enter the "Flow Velocity (m/s)" of the water moving through the pipe.
Click "Calculate" to see the resulting flow rate in litres per minute and US gallons per minute.
Frequently Asked Questions
It uses the continuity equation: Flow Rate = Cross-Sectional Area × Velocity, where the area is calculated from the pipe's internal diameter (Area = π × r²).
Cold water supply pipes are usually designed for 1.0-2.0 m/s, while hot water pipes are often kept lower (around 0.9-1.2 m/s) to reduce noise and heat loss.
It should be the internal (bore) diameter of the pipe, since that's the actual area through which water flows.
Yes, it's completely free and requires no sign-up.