Pounds per hour (lb/hr) is a mass flow rate, whereas standard cubic feet per hour (SCFH) is a volumetric flow rate measured at a specific temperature and pressure. Therefore, the density of the fluid being measured as well as the temperature and pressure at which the measurement is taken must be known before the conversion can be performed.

Calculate the density of the fluid. The density of fluids varies depending on the temperature and pressure. Liquid densities are generally less sensitive to temperature and pressure than gases, but for both fluid types the density must be found under specific conditions. Once the density is known, convert the units to pounds per cubic foot (lb/ft^3) using the appropriate conversion factors.

If the fluid is a liquid, the flow rate in standard cubic feet per hour and absolute cubic feet per hour are identical. For gases, the flow rate must be converted to standard units using the standard and absolute air pressure (Pstd and Pact) and temperature (Tstd and Tabs). The relative humidity (phi) and saturation pressure (Psat) at absolute conditions must also be known. The flow rate in standard cubic feet per hour is then the absolute flow rate multiplied by the quantity ((Pact - Psat*phi)*(Tact/Tstd))/Pstd.