Duct sizing calculator

Size HVAC ducts for optimal airflow and velocity

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The Duct Sizing Calculator helps HVAC engineers and contractors determine appropriate duct dimensions based on airflow requirements and velocity limits. Proper duct sizing ensures efficient air distribution while minimizing pressure drop and noise.

Ducts that are too small create excessive pressure drop and noise, while oversized ducts waste materials and space. The calculator uses the fundamental airflow equation to determine optimal duct sizes for both round and rectangular configurations.

Enter your airflow requirements (CFM) and desired velocity to instantly calculate required duct diameter or rectangular dimensions. The tool helps balance airflow efficiency with practical installation constraints, supporting both round and rectangular duct designs.

Features

Everything you need

Round and rectangular duct sizing
Airflow and velocity calculations
Visual duct cross-section diagrams
CFM and metric unit support
Velocity limit recommendations
Aspect ratio optimization
Pressure drop considerations

How it works

Simple workflow

1
Enter required airflow (CFM or m³/h)
2
Specify desired velocity (fpm or m/s)
3
Choose round or rectangular duct shape
4
View calculated duct dimensions
5
Adjust velocity to optimize size

Use cases

Built for real-world scenarios

HVAC system design

Size ductwork for new HVAC installations to ensure proper airflow distribution.

System retrofitting

Determine duct sizes when upgrading or modifying existing HVAC systems.

Energy efficiency

Optimize duct sizes to minimize pressure drop and reduce fan energy consumption.

Noise control

Size ducts to maintain velocities within acceptable noise limits for occupied spaces.

FAQ

Frequently asked questions

Duct area = Airflow ÷ Velocity. For round ducts: Diameter = √(4 × Area ÷ π). For rectangular ducts, choose dimensions that provide the required area while fitting available space.

Recommended velocities: 600-900 fpm for main ducts, 400-600 fpm for branch ducts, 300-500 fpm for terminal runs. Higher velocities increase noise and pressure drop.

Round ducts are more efficient (less pressure drop per area) and easier to seal, but rectangular ducts fit better in tight spaces. Rectangular ducts should maintain aspect ratios under 4:1 for efficiency.

Smaller ducts create higher velocities and greater pressure drop, requiring more fan energy. Properly sized ducts balance airflow needs with energy efficiency and noise control.

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We're always looking to improve our calculators. If you have ideas for new features, improvements, or found something that could work better, we'd love to hear from you.

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