Duct Size Calculator

Size rectangular or round ducts for HVAC airflow.

ROUND DUCT
— in
diameter

Usage Tip

Round ducts move air most efficiently. Common sizes step through 6, 8, 10, 12, 14, 16 in.

THE MATH
d = √(4A ÷ π),   A = CFM ÷ FPM × 144
Where: A = duct cross-section (sq in)  •  FPM = air velocity
• Round ducts are the most efficient
• Size up to the next standard size
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The calculators and tools on Formula Factory are provided for general guidance and informational purposes only. Results are estimates based on standard formulas and the values you enter — they do not constitute professional engineering, electrical, or architectural advice. Always verify calculations with a qualified professional before making decisions for any safety-critical, code-compliance, or commercial application. Formula Factory makes no representations or warranties as to the accuracy or completeness of any result, and accepts no liability for errors, omissions, or any outcomes arising from reliance on this information.