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About HVAC Duct CFM Calculator
Calculate volumetric airflow rate in cubic feet per minute (CFM) for round and rectangular HVAC ducts using internal cross-sectional area and air velocity.
Inputs and calculations stay in your browser and require no signup.
How the calculation works
Planning calculation
Round area (sq ft) = π × (diameter / 2)² ÷ 144; Rectangular area (sq ft) = (width × height) ÷ 144; Airflow CFM = duct area (sq ft) × velocity (ft/min).
How to use this calculator
- 1Enter measured project dimensions and quantities.
- 2Adjust material, spacing, waste or density assumptions to match the project.
- 3Review the live planning quantities.
- 4Round purchasing quantities appropriately and confirm field conditions.
Understanding the result
Airflow is directly proportional to internal cross-sectional duct area and air velocity. Branch ducts typically operate at 600 to 800 FPM, while main supply trunks run at 700 to 1,000 FPM.
Results use the units shown beside each field and output.
Important limitations
- Calculates theoretical airflow from cross-sectional area and air velocity only (Q = A × V).
- Does not size complete duct systems, calculate static pressure drops, friction loss, equivalent fitting lengths, or fan curves.
- Use ACCA Manual D or ASHRAE fundamentals for comprehensive HVAC residential and commercial duct sizing.
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Frequently asked questions
What is CFM in HVAC?
CFM stands for Cubic Feet per Minute, measuring the volume of conditioned air circulated through ducts and spaces.
What is standard residential duct air velocity?
Residential branch ducts typically aim for 600 to 700 FPM to keep air noise quiet, while supply trunks run 700 to 900 FPM.
How do I convert round duct diameter to CFM?
Find the cross-sectional area in square feet: π × (diameter ÷ 2)² ÷ 144, then multiply by air velocity in feet per minute (FPM).
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