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Duct Size Calculator

Calculate the proper duct size for your HVAC system using airflow (CFM), velocity and friction rate to achieve efficient air distribution.

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Duct Size Calculator
Input Parameters
CFM
FPM
Recommended 500–900 FPM
Calculation Results
Recommended Duct Size
12" × 8"
Width × Height (inches)
Actual Velocity
598
FPM
Friction Rate
0.078
in.wg /100 ft

Detailed Results

Airflow 1200 CFM
Duct Type Rectangular
Duct Size 12" × 8"
Actual Area 0.67 ft²
Actual Velocity 598 FPM
Equivalent Diameter 9.8"
12" 8"
System Check

This duct size is within the recommended airflow range.

About This Calculator

This HVAC Duct Size Calculator estimates the recommended duct dimensions using airflow (CFM), air velocity and friction rate. Proper duct sizing improves comfort, reduces noise, lowers energy consumption and helps HVAC systems operate efficiently.

The results are intended for preliminary HVAC design and should be verified against ACCA Manual D or applicable local design standards before installation.

Recommended Air Velocity

Application Velocity (FPM)
Main Supply 700 – 900
Branch Supply 500 – 700
Main Return 600 – 900
Branch Return 400 – 600

Typical Friction Rates

System Type in.wg /100 ft
Residential 0.08
Commercial 0.10
High Velocity 0.12
Low Noise Design 0.06

HVAC Design Tips

  • Keep duct runs as short and straight as possible.
  • Minimize sharp elbows and sudden transitions.
  • Seal all duct joints to reduce air leakage.
  • Insulate ducts installed in unconditioned spaces.
  • Balance airflow to each room after installation.
  • Verify total external static pressure during commissioning.