How to Calculate Duct Sizing
What is Duct Sizing?
Duct sizing balances airflow velocity, pressure drop, and cost. Proper sizing ensures system performance and efficiency.
Step-by-Step Guide
- 1Use friction loss tables or duct calculators
- 2Recommended velocity: main ducts 4-6 m/s, branches 3-4 m/s
- 3Size ducts for <25 Pa/100m static pressure drop
Worked Examples
Input
CFM 2500, round duct, 0.15 mmWG friction
Result
Diameter ≈ 500 mm for low-loss duct design
Provides good velocity
Common Mistakes to Avoid
- ✕Using velocity too high (noise and friction)
- ✕Neglecting pressure drop from fittings and dampers
Frequently Asked Questions
Why not use smallest possible ducts?
Small ducts have high velocity, high friction loss, and noise. Oversizing slightly saves fan energy.
How does duct insulation affect sizing?
Insulation doesn't affect sizing directly, but better insulation reduces equipment sizing needs.
Ready to calculate? Try the free Duct Sizing Calculator
Try it yourself →