Duct Size Calculator
Calculate HVAC duct size based on CFM airflow requirements. Find optimal round and rectangular duct dimensions for heating and cooling systems.
How to Use This Calculator
Size HVAC ductwork for optimal airflow:
- Enter the required airflow in CFM for your duct run
- Select target velocity (lower = quieter, higher = smaller ducts)
- Choose your duct type (round, rectangular, flex, or duct board)
- Select friction rate based on system design
- Review recommended duct size and rectangular equivalents
Why This Matters
Properly sized ductwork is critical for HVAC system performance. Undersized ducts restrict airflow, causing poor heating/cooling, system strain, higher energy bills, and premature equipment failure.
- Calculate optimal duct diameter for CFM requirements
- Get rectangular duct equivalent dimensions
- Check velocity for noise considerations
- Estimate friction loss per 100 feet
Worked Examples
Main Trunk Duct
1,200 CFM at 800 FPM for main supply
Calculation: 1200 ÷ 800 = 1.5 sq ft = 216 sq in area needed
Result: 18 inch round or 20×12 rectangular
Bedroom Branch Duct
150 CFM at 600 FPM for quiet operation
Calculation: 150 ÷ 600 = 0.25 sq ft = 36 sq in
Result: 7 inch round or 8×6 rectangular
Common Mistakes to Avoid
- Using same size throughout instead of reducing at branches
- Forgetting flex duct needs 1-2 inches larger than rigid
- Ignoring equivalent length of fittings (90° elbow = 10-15 ft)
- Installing rectangular ducts with aspect ratio over 4:1
- Not accounting for filter and coil pressure drops