Duct Calculator: Efficient Sizing for HVAC Systems & Ductwork

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:

  1. Enter the air flow rate in cubic feet per minute (CFM).
  2. Enter the desired velocity of the air in feet per minute (FPM).
  3. 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.