CFM stands for cubic feet per minute, and the basic formula for calculating CFM is very simple:
CFM = Velocity (ft/min) × Area (ft²)
This relationship comes from the definition of volume flow: the amount of air that passes through a cross section per unit time. In practical fan and ventilation work, you either measure the air velocity and duct size directly or convert from metric units used in many industrial and mining projects.
To use the formula, you first determine the internal duct area where you are measuring. For a rectangular duct, area in ft² is width × height (both in feet). For a round duct, area is π × (diameter/2)². For example, a 2 ft by 1.5 ft duct has an area of 3 ft², and a 24 inch (2 ft) diameter duct has an area of about 3.14 ft².
Next, you measure air velocity using a vane anemometer, hot wire probe or Pitot tube. Most instruments can display velocity directly in ft/min, but if you measure in m/s you convert using 1 m/s ≈ 196.85 ft/min. Then you multiply the average velocity by the area to obtain CFM. If the velocity varies across the duct, you should take several readings and average them to improve accuracy.
In many industrial and mining calculations, airflow is initially expressed in m3/h. You can convert between CFM and m3/h using the approximate relationships:
- 1 CFM ≈ 1.699 m3/h
- 1 m3/h ≈ 0.5886 CFM
So if a mine ventilation design calls for 60,000 m3/h through a duct, that is about 35,300 CFM. These conversions help when you are comparing fan catalog data, which may be published in different units.
CFM can also be estimated from fan curves and system resistance. Once you know the operating point on the curve in metric units, you convert the flow rate into CFM using the factors above. However, the underlying volume flow is still based on the same principle: velocity multiplied by area.
In summary, the formula for calculating CFM is CFM = velocity × area, with velocity in feet per minute and area in square feet. In industrial and mining ventilation, this simple relationship, supported by proper measurements and unit conversions, underpins most airflow calculations.
