Duct Size Calculator
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Duct area from CFM ÷ velocity — round and square equivalents.
Rates last reviewed: July 2026
Airflow
Round duct
10.2 in
Square ~9.1 × 9.1 in
- Area
- 82.3 sq in
- Square equivalent
- 9.1 in side
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Duct cross-section
Airflow (CFM) divided by velocity (feet per minute) gives required duct cross-sectional area in square feet. Convert to round diameter or square sides for layout planning.
Lower velocity is quieter but needs larger ducts; supply trunks often target 700–900 fpm, branches 500–700 fpm — verify with your design standards. This is a geometry calculator, not full duct design.
Example: 400 CFM at 700 fpm → area ≈ 0.571 sq ft → round duct about 10.2 in diameter.
Common questions
What velocity should I use?
Residential supply often 600–900 fpm in trunks, lower in branches. Higher velocity increases noise and friction losses.
Does this include fittings?
No. Elbows and transitions add equivalent length and pressure drop — size ducts with proper HVAC design tools for installs.