Duct Size Calculator
Calculate round duct diameter and rectangular equivalent for any CFM airflow requirement. HVAC duct sizing using velocity and equal friction methods.
CFM
ft/min
Round Duct Diameter
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Rectangular Equiv. Width —
Rectangular Equiv. Height —
Actual Velocity —
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CFM
ft/min
Round Diameter
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Rectangular Equiv. —
Actual Velocity —
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CFM
ft/min
ft
ft
in WC
Duct Sizing
Main Trunk Diameter —
Required Friction Rate —
Total Equiv. Length —
Trunk Velocity —
Distribution
Register Size (est.) —
How to Use This Calculator
- Enter the required CFM for the room or zone.
- Set max air velocity (default 900 ft/min for residential).
- Results show round duct diameter and nearest rectangular equivalent.
Formula
Area (ft²) = CFM ÷ Velocity
Round diameter (in) = √(Area × 4 / π) × 12
Round up to next standard size: 4, 5, 6, 7, 8, 9, 10, 12, 14, 16 in.
Example
Example: 200 CFM at 900 ft/min → area = 0.222 ft² → diameter = 6.4 in → use 7-inch round duct.
Frequently Asked Questions
- CFM (cubic feet per minute) is the volume of air delivered through a duct or register per minute. It is the primary measure of airflow capacity in HVAC systems. A typical bedroom requires 50–150 CFM for adequate conditioning. A living room may need 200–400 CFM. The required CFM for a room is calculated as: CFM = (Room Volume in cubic feet × ACH) ÷ 60, where ACH is the number of air changes per hour recommended for that space type. Residential systems typically provide 4–8 air changes per hour depending on the application.
- Residential supply ducts should maintain air velocity between 600–900 feet per minute (ft/min) for comfort and noise control. Below 600 ft/min, airflow can stagnate; above 1,000 ft/min, turbulence noise becomes audible at registers. Return air ducts are quieter and can run at 600–700 ft/min. The default in this calculator is 900 ft/min, which is the upper limit for most residential installations. Commercial and industrial ductwork can run 1,000–2,000 ft/min. Reducing velocity requires larger duct diameters but results in quieter operation.
- The formula for round duct diameter based on airflow and velocity is: Diameter (in) = √(144 × CFM ÷ (0.7854 × Velocity_ft_per_min)) × 12. First calculate the required cross-sectional area in sq ft: Area = CFM ÷ Velocity. Then find the diameter: D = √(4 × Area ÷ π) × 12. For example, 200 CFM at 900 ft/min: Area = 200 ÷ 900 = 0.222 sq ft. D = √(4 × 0.222 ÷ 3.14159) × 12 = 6.4 in. Round up to the next standard size: 7-inch duct. This calculator solves this automatically.
- A rectangular duct has the same airflow capacity as a round duct when their hydraulic diameters are equal. The hydraulic diameter of a rectangular duct is: Dh = 1.30 × (a × b)^0.625 ÷ (a + b)^0.25, where a and b are the duct dimensions. Common equivalents: 6-inch round ≈ 6×5 or 8×4 rectangular. 8-inch round ≈ 8×7 or 10×6 rectangular. 10-inch round ≈ 10×9 or 12×8 rectangular. Square ducts of the same nominal size (6×6, 8×8, etc.) are slightly larger than the equivalent round duct.
- ACH (air changes per hour) is the number of times the total room air volume is replaced per hour. Recommended values by space type: bedrooms and living rooms 4–6 ACH, home offices 4–6 ACH, kitchens 7–8 ACH (for cooking odors and heat), bathrooms 6–8 ACH (for humidity control), conference rooms 6–8 ACH. Formula: Required CFM = (Room length × width × height × ACH) ÷ 60. For example, a 12 × 14 × 9 ft bedroom at 6 ACH = 12 × 14 × 9 × 6 ÷ 60 = 151 CFM.