Calculating the CFM of an axial fan means determining how many cubic feet of air per minute the fan is actually moving under real operating conditions. The most straightforward method combines velocity measurements with the area of the duct or opening where the fan is installed.
The core relationship is:
CFM = Velocity (ft/min) × Area (ft²)
For an axial fan installed in a cylindrical duct, you first calculate the internal cross-sectional area. If the duct diameter is D (in feet), the area is:
A = π × (D / 2)²
For a rectangular opening, the area is simply width × height (in feet). Once you know the area, you measure the average air velocity in the duct using a vane anemometer, hot wire probe or Pitot tube, typically in ft/min.
Because axial fans often have non-uniform velocity profiles, it is important to take multiple readings across the duct. You might measure at several points along the diameter or in a grid pattern, then average the values to obtain a representative velocity. Multiplying this average velocity by the duct area gives the airflow in CFM.
In industrial and mining applications, you also compare your measurement with the manufacturer’s axial fan curve. The fan curve shows CFM versus static pressure at a given speed (and blade angle if adjustable). By measuring the fan’s static pressure rise and locating that pressure on the curve, you can read off the approximate CFM. Using both velocity measurements and the fan curve together provides a more reliable picture of real performance.
If your instruments measure in metric units (m/s and m2), you first calculate airflow in m3/s and then convert to CFM using:
- 1 m3/s ≈ 2118 CFM
- 1 CFM ≈ 0.0004719 m3/s
For large axial fans in tunnels and mines, the same principle applies but the area is the roadway or shaft cross section instead of a metal duct. Velocity readings are taken at multiple points across the opening, averaged, and multiplied by the cross-sectional area to obtain CFM or m3/s.
In summary, to calculate the CFM of an axial fan you determine the cross-sectional area where the air flows, measure average air velocity, and multiply velocity by area. Fan curves and pressure measurements are then used to confirm that the calculated airflow matches the fan’s expected performance in the system.
