HVAC Duct Size Calculator
Properly sized ductwork ensures your HVAC system delivers the right amount of air to each room. Undersized ducts create noise, reduce airflow, and waste energy. Enter your CFM requirements to find the right duct diameter.
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Results
Enter your measurements to see results
How to Use This Calculator
- Enter the required CFM for the room or branch.
- Select the appropriate velocity (lower for bedrooms, higher for trunk lines).
- Optionally enter system tonnage to see total system CFM.
- Review the recommended duct diameter.
How Duct Size Calculation Works
Air volume (CFM) divided by velocity (feet per minute) gives the required duct cross-sectional area. The calculator converts this area to a round duct diameter. Lower velocities mean quieter operation but require larger ducts. The standard rule is 400 CFM per ton of cooling capacity for the total system — individual room CFM is proportional to the room's share of the total cooling load.
Formula
Duct Area (sq in) = CFM × 144 ÷ Velocity (FPM). Diameter = √(4 × Area ÷ π). System CFM = Tons × 400.
Frequently Asked Questions
What CFM per ton do I need?
The standard rule is 400 CFM per ton of cooling capacity. A 3-ton system needs about 1,200 CFM total, distributed across rooms proportionally.
What size duct for a 12×12 bedroom?
A 12×12 bedroom typically needs 80–120 CFM, which requires a 6-inch round duct at standard branch velocity (650 FPM).
Can I use flex duct instead of sheet metal?
Yes, but flex duct has 20–30% more resistance than rigid metal for the same diameter. You may need to upsize by one inch to compensate.