The continuity equation
Volumetric flow rate Q equals the cross-sectional area A of the pipe multiplied by the average fluid velocity v. Mass flow rate adds fluid density to the equation: ṁ = ρAv. These relationships assume steady, incompressible flow with uniform velocity across the cross-section.
In practice, velocity varies across the pipe profile (faster in the centre, slower near the walls), so the "velocity" used here is the mean velocity. For engineering calculations, this simplification is standard.
Q = A × v
Q in m³/s, A in m², v in m/s.
ṁ = ρ × A × v
Mass flow rate in kg/s, where ρ is fluid density in kg/m³.