Duct design
Duct Cross-Sectional Area Calculator
Calculate the face area of a round duct from its inside diameter.
Calculator
How to use
Enter the duct's inside diameter. Use face area with CFM to check velocity.
Formula
Area = π × (D ÷ 12)² ÷ 4
Variables
D is diameter in inches; area is reported in square feet and square inches.
Example
A 12 in round duct has an area of about 0.785 ft² or 113.1 in².
Result interpretation
Interior dimensions, liners, and fittings affect the effective flow path.
FAQ
Can I use outside duct dimensions?
Use inside dimensions for airflow area whenever available.