Mass flow rate is the mass of fluid that passes through a certain face per unit time.

When fluid density is constant, mass flow rate can be expressed with the following equation:

where;
 : Mass flow rate [kg/s]
 ρ  : Fluid density [kg/m3]
 A  : Area of the face through which flow passes through[m2]
 v  : Average fluid velocity of flow perpendicular to the face through which fluid passes [m/s]

The equation above can also be expressed using volumetric flow rate Q [m3/s] as below: