Use our duct calculator tool to accurately determine the dimensions and air flow for your HVAC ductwork.
How to use the Duct Calculator
To calculate the area and diameter of a duct, follow these simple steps:
- Enter the air flow rate in cubic feet per minute (CFM).
- Enter the desired velocity of the air in feet per minute (FPM).
- Click the “Calculate” button to see the result.
How the Calculation Works
The calculator uses the formula to find the cross-sectional area of the duct:
Area = Flow Rate / Velocity
Then it calculates the diameter of a round duct with that area by:
Diameter = sqrt((4 * Area) / pi)
Note that this calculator assumes a round duct.
Limitations
The calculator does not account for different shapes of ducts and is based on standard air conditions and a circular duct assumption. It may not be accurate in calculating for ducts that are rectangular or have complex shapes or in extreme temperature and pressure conditions.