HVAC Duct Size Calculator
Size your HVAC ductwork correctly based on CFM airflow requirements. Enter the required CFM and desired velocity to get round duct diameter and equivalent rectangular duct dimensions. Proper duct sizing prevents noise, ensures comfort, and maximizes system efficiency.
Enter Airflow Requirements
Full Breakdown
Required CFM
Target Velocity
Round Duct Diameter
Duct Area
Rectangular Equivalent
Actual Velocity
Noise Level
How to Use This Calculator
- Enter the required CFM for the room or duct branch (use 1 CFM per sq ft as a rule of thumb for residential).
- Select a target velocity — 600–700 FPM for residential, 900+ for commercial.
- Optionally enter a rectangular duct height to get width for that height.
- Click "Calculate Duct Size" for round and rectangular options.
- Keep velocity under 900 FPM in residential to minimize noise.
The Formula
Duct Area (sq in) = CFM ÷ Velocity (FPM) × 144
Round Diameter = √(4 × Area ÷ π)
The 144 converts from sq ft to sq in. Round up to the nearest standard duct size (6", 7", 8", 10", 12", 14", etc.).
Rectangular Width = Area ÷ Height
For rectangular equivalents, the cross-sectional area must match the round duct area.