Pipe Flow Rate Calculator

About this calculator

The volume of fluid moving through a pipe per unit time — its volumetric flow rate — follows the continuity equation from fluid mechanics: Q = A × v, where Q is flow rate, A is the pipe’s cross-sectional area (calculated from its inner diameter as A = π × (d/2)²), and v is the average flow velocity. Because area scales with the square of the diameter, even a modest increase in pipe diameter dramatically increases the flow rate a pipe can carry at the same velocity.

Plumbers and mechanical/civil engineers use this relationship to size pipework for a required flow — undersized pipe diameter forces higher velocity to deliver the same flow, which increases friction losses, pressure drop and noise, while oversized pipe wastes material and can allow sediment to settle at low velocities. It’s the same equation behind sizing everything from domestic water supply lines to industrial process piping and irrigation systems.

This calculator takes your pipe’s inner diameter and flow velocity and returns the volumetric flow rate in litres per second and cubic metres per hour.