"Mass flow" is the movement of a mass of material; often it is expressed numerically in pounds. "Volumetric flow" is the movement of a volume of material; often it is expressed numerically in cubic feet.
Normally when calculating flows, materials that are gases or liquids are considered. The density of a gas or liquid is what relates the mass flow to the volumetric flow. Density is the mass (or weight) of material contained in a given volume; often it is expressed numerically in pounds per cubic foot.
Look up the density of your material of interest in pounds per cubic foot. Links to material densities are in the Resources section.
Choose a mass flow for your material, in pounds, that you want to convert to a volumetric flow.
Divide the mass flow by the density. The result is the volumetric flow, expressed as cubic feet of material. An example is: 100 pounds (mass flow) / 10 pounds per cubic foot (density) = 10 cubic feet (volumetric flow).
Mass flow is normally understood as "mass flow rate," the mass of a material crossing a fixed point in a given amount of time. Mass flow rate is often expressed as pounds per hour. Volumetric flow is normally understood as "volumetric flow rate," the volume of a material crossing a fixed point in a given amount of time. Volumetric flow rate is often expressed as cubic feet per hour.
Dividing the mass flow by the time, in hours, gives the mass flow rate in pounds per hour. Dividing the volumetric flow by the time, in hours, gives the volumetric flow rate in cubic feet per hour.
Be consistent with your use of measurement units in flow calculations. If you use a mass expressed in pounds, a density expressed in pounds per cubic foot and a time expressed in hours to calculate a volumetric flow rate, for example, then the result must be expressed as cubic feet per hour. You can convert the result to other measurement units if you wish. For example: a flow rate expressed as cubic feet per hour can be converted to liters per minute or cubic meters per hour. To convert flows and densities to different measurement units, see the Resources section.