Understanding the CFM Formula
The amount of air a duct can carry is its volume, which is determined by its cross-sectional area and the speed (velocity) of the air moving through it.
The formula is: CFM = Area (in sq ft) × Velocity (in FPM). This calculator simplifies the process by taking the area in square inches and converting it to square feet (dividing by 144) as part of the calculation.
How to Use the Calculator
- Calculate the cross-sectional area of your duct in square inches. For a round duct, Area = π × radius². For a rectangular duct, Area = Length × Width.
- Enter the velocity of the air in Feet per Minute (FPM). This can be measured with an anemometer. A typical residential system has a velocity of 700-900 FPM in main ducts.
- Click "Calculate CFM" to see the total airflow volume your duct is carrying.