Continuity Equation Calculator

Solve for flow rate, velocity, or cross-sectional area using Q = A × V for incompressible flow.

Fluid Mechanics I

Inputs

Enter any two of the three quantities — the third will be calculated.

Flow rate Q (m³/s)

Mean velocity V (m/s)

Pipe diameter D (m)

Cross-sectional area A (m²)

Theory

Main Equation:

Q = A × V

Where:

Q = volumetric flow rate [m³/s]
A = cross-sectional area [m²] = π D² / 4 for a circular pipe
V = mean flow velocity [m/s]

For incompressible flow, mass is conserved — ρAV = constant along any streamtube. Since ρ is constant, AV = constant: a smaller cross-section forces higher velocity.