Duct Sizing Calculator
Calculate round and rectangular duct dimensions from CFM
Round Duct
Rectangular Duct
Airflow (CFM)
Cubic feet per minute
Design Velocity
600 FPM — Quiet / bedroom
700 FPM — Low noise
800 FPM — Standard
900 FPM — Main trunk
1000 FPM — High velocity
Feet per minute
Aspect Ratio (W : H)
1:1 — Square
1.5:1 — Common
2:1 — Wide
3:1 — Flat
4:1 — Very flat
Calculate Duct Size
Recommended duct size
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Cross-section area
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Actual velocity
Quiet (600)
High (1000)
Sizes rounded up to standard dimensions. See ACCA Manual D for full duct design.
Provided by
Comfortable HVAC Services
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