Calculate CFM from orifice differential pressure or a K-factor, reverse-solve pressure, and review beta correction, velocity, mass flow and SI airflow results.

Match the model to the pressure measurement. Orifice flow assumes constant density; the K-factor must match the actual equipment and taps. These estimates do not certify a flow-meter installation.

Conventional inH₂O: 1 = 249.08891 Pa.

Use the coefficient for the actual device and pressure taps.

Default: 1.204 kg/m³, approximately dry air at 20°C and 101.325 kPa. Enter the actual value.

CFM From Differential Pressure Formula

The following two example problems outline the steps and information needed to calculate the CFM From Differential Pressure.

CFM ≈ 4005 × A × sqrt(velocity pressure)

Variables:

  • CFM is the CFM From Differential Pressure (ft^3/min)
  • dP is Pitot velocity pressure (total minus static), in conventional inches of water; arbitrary fan or filter pressure drop is unsuitable.
  • A is the cross-sectional area (ft^2) 

For the Pitot path, the approximate 4005 factor assumes standard air density of 0.075 lb/ft³ and a representative mean velocity. The calculator instead uses the entered operating density: Q = A × √(2ΔP/ρ), converted from m³/s to CFM. For a traverse, average velocities rather than pressures.

How to Calculate CFM From Differential Pressure?

Select Orifice airflow for Q = Cd × A × √(2ΔP/ρ)/√(1−β⁴), using bore area, β = bore/pipe diameter and the correct discharge coefficient and meter taps. Free-opening mode omits the beta term. Select Orifice pressure for its inverse. These are constant-density estimates, not ISO 5167 certification. Equipment K airflow uses CFM = K × √(ΔP in inH₂O), with the manufacturer’s calibration. Use Pitot airflow for the velocity-pressure formula below.

  1. First, measure representative Pitot velocity pressure (total minus static), in conventional inH₂O.
  2. Next, determine duct cross-sectional area (ft²) and operating air density.
  3. Select Pitot airflow and enter the measured area, pressure and density.
  4. Finally, calculate the CFM From Differential Pressure.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

differential pressure (W.C.) = .50

cross-sectional area (ft^2) = 2.5

For A = 2.5 ft², velocity pressure = 0.50 inH₂O and density = 0.075 lb/ft³, the density-aware result is about 7,086.184154 CFM. The rounded 4005 shortcut gives about 7080 CFM.