Calculate CFM from differential pressure across an orifice, with an equipment K-factor or a pitot tube, or find the pressure needed for a target airflow.

Conventional inHโ‚‚O: 1 = 249.08891 Pa.

0.62 is typical for a sharp-edged orifice. Use your device’s coefficient if you know it.

1.204 kg/mยณ is standard dry air at 20ยฐC and sea-level pressure. Change it for hot, cold or high-altitude air.

Optional settings

CFM From Differential Pressure Formula

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

CFM โ‰ˆ 4005 ร— A ร— โˆš(velocity pressure)

Variables:

  • CFM is the CFM From Differential Pressure (ft3/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 (ft2) 

For the Pitot path, the approximate 4005 factor assumes standard air density of 0.075 lb/ftยณ and a representative mean velocity. The calculator instead uses the entered operating density: Q = A ร— โˆš(2ฮ”P/ฯ), converted from mยณ/s to CFM. For a traverse, average velocities rather than pressures.

How to Calculate CFM From Differential Pressure?

Select Orifice airflow for Q = Cd ร— A ร— โˆš(2ฮ”P/ฯ)/โˆš(1โˆ’ฮฒโด), using bore area, ฮฒ = 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 ร— โˆš(ฮ”P in inHโ‚‚O), with the manufacturerโ€™s calibration. Use Pitot airflow for the velocity-pressure formula below.

  1. First, measure representative Pitot velocity pressure (total minus static), in conventional inHโ‚‚O.
  2. Next, determine duct cross-sectional area (ftยฒ) 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 (ft2) = 2.5

For A = 2.5 ftยฒ, velocity pressure = 0.50 inHโ‚‚O and density = 0.075 lb/ftยณ, the density-aware result is about 7,086.184154 CFM. The rounded 4005 shortcut gives about 7080 CFM.