Duct Size Calculator
Size HVAC ducts using velocity and friction methods
Design Parameters
Velocity Guidelines:
• Main Ducts: 700-900 FPM
• Branch Ducts: 600-700 FPM
• Supply Outlets: 500-600 FPM
• Return Grilles: 300-500 FPM
Duct Size Results
Round Duct (Velocity Method)
10" diameter
Calculated: 9.0" | Actual velocity: 733 FPM
Rectangular Duct
12" × 6"
Calculated: 11.3" × 5.7"
Equivalent round: 9.1" diameter
Round Duct (Friction Method)
6" diameter
Calculated: 4.1"
Performance Data
Minimum Area:64.0 sq inPressure Drop:8.00 in. w.g./100ftNoise Level:Medium
Standard Round Duct Sizes:
4", 5", 6", 7", 8", 9", 10", 12", 14", 16", 18", 20", 24"
Tip: Keep aspect ratio ≤ 4:1 for rectangular ducts