Mass flow rate is the rate at which the mass of a substance passes through a given surface. Similarly, the volume flow rate is the rate at which the volume of a fluid passes through a given surface. These measures are used extensively in fluid dynamics, and it's frequently necessary to convert these measures of flow. Note that both liquids and gases are considered to be fluids in the context of fluid dynamics.
Examine the units of measure for mass flow rate. The mass flow rate measures mass per unit time, so the standard unit of measure for the mass flow rate is kilograms per second (kg/s).
Look at the units of measure for volume flow rate. The volume flow rate measures volume per unit time so the standard unit of measure for the volume flow rate is cubic meters per second (m^3/s).
Relate the mass of a fluid to its volume with density. The density of a material is defined as d = m/v, where d is its density in kg/m^3, m is its mass in kg and v is its volume in m^3. Therefore, v = m/d.
Calculate the mass flow rate. This is given by the equation M = dsA, where M is the mass flow rate, d is the density of the fluid and s is the velocity of the fluid through the surface area A.
Convert the mass flow rate to the volume flow rate. Since v = m/d from Step 3, we can state that V = M/d, where V is the volume flow rate and M is the mass flow rate. Note that this equation also gives us V = M/d = dsA/d = sA, which allows us to calculate V directly from the velocity of the fluid and the area of the given surface.