Flow Rate Calculator

Calculate volumetric flow rate in a pipe from its diameter and fluid velocity, using Q = A x V.

⏱ Updated: 18 Sep 2026

Calculator

Free flow rate calculator: find volumetric flow rate in a pipe from its diameter and the fluid velocity, using Q = A × V.

Flow rate follows the continuity equation, Q = A × V — cross-sectional pipe area times fluid velocity. Switching units re-derives the pipe diameter and velocity, not just the labels.

Enter a pipe's inside diameter and the fluid's velocity to calculate volumetric flow rate in GPM or L/min.

How to Use the Flow Rate Calculator

Switch the unit toggle to US or Metric first, then enter the pipe's inside diameter and the fluid's velocity — the toggle re-derives both fields rather than just relabeling them, so switching mid-calculation carries the numbers over instead of resetting them.

Volumetric flow rate through a pipe is the cross-sectional area the fluid is moving through, multiplied by how fast it's moving.

A = π × (diameter ÷ 2)²
Q = A × V

Double a pipe's diameter and its cross-sectional area quadruples, not doubles — area scales with the square of radius, so flow capacity at a given velocity grows much faster than the diameter number alone implies. That's the whole reason a 1-inch supply line can't just be swapped for "half of" a 2-inch main and expected to carry half the water.

Results show in both GPM and L/min regardless of which unit system is active on the inputs, alongside the pipe's cross-sectional area, so a US-measured pipe can be compared directly against a metric spec sheet without leaving the page.