This advanced HVAC duct sizing calculator helps engineers calculate circular and rectangular duct sizes using either Imperial (CFM, FPM) or Metric (m³/h, m/s) units. It is suitable for AHU, FCU, and ventilation duct design.
Calculation Result
Enter values and click calculate.
How This Calculator Works
The calculator first determines duct area using:
Area = Airflow ÷ Velocity
For circular ducts:
Diameter = √(4 × Area ÷ Ï€)
For rectangular ducts, width and height are calculated using the selected aspect ratio.
Recommended Velocity Values
- Main supply ducts: 800 – 1200 FPM (4 – 6 m/s)
- Branch ducts: 600 – 900 FPM (3 – 4.5 m/s)
- Return air ducts: 500 – 800 FPM (2.5 – 4 m/s)
Important Notes
- This calculator is for preliminary HVAC design
- Final duct sizing should follow ASHRAE / SMACNA standards
- Pressure drop and noise criteria must be checked separately
Related Guide
👉 How to Calculate Duct Size in HVAC (Step-by-Step Guide)
