Duct geometry
Duct Cross-Sectional Area Calculator
Calculate round or rectangular duct face area from actual inside dimensions.
Round duct area
Use inside diameter for the face area available to airflow.
Rectangular duct area
Use actual inside width and height, excluding any reduced clear opening.
How to use
- Enter the actual inside dimensions of the selected duct.
- Use the area with assigned CFM to calculate preliminary face velocity.
- Check fittings, liners, dampers, transitions, and the full system separately.
Formula
Rectangular area (ft²) = width (in) × height (in) ÷ 144
Velocity (FPM) = CFM ÷ face area (ft²)
Example
A 12 in round duct has about 0.785 ft² (113.1 in²) of face area. An 18 in × 8 in rectangle has 1.00 ft² (144 in²) of face area.
Result interpretation
Face area is a geometry value. It does not describe friction loss, available fan pressure, fitting losses, effective flow area at a grille, or air distribution. Use the actual inside opening and check the selected configuration with the whole system.
Use area with a known airflow
Enter the run CFM and actual size in the Duct Velocity Calculator. Use the Duct Size Calculator when you need a preliminary size from CFM and a documented velocity basis.
Frequently asked questions
Can I use outside duct dimensions?
Use inside dimensions for airflow face area whenever available. Liners or reduced openings can make the usable area smaller.
Does equal area mean equal friction loss?
No. Duct shape, roughness, fittings, length, and air conditions affect resistance.