Duct Size Calculator
Calculate optimal duct diameter and dimensions
Why this tool matters
Solve the calculation or lookup now, then keep the answer attached to the installed base so it can support maintenance follow-up, warranty timing, equipment history, and replacement conversations later.
Next best step
Use the result in the field, then start a free AssureStep account when you want the context to stay visible after this visit.
Velocity Guidelines
Main Supply Ducts
700-900 FPM
Higher velocity = smaller ducts, more noise
Branch Ducts
500-700 FPM
Lower velocity = quieter operation
Return Ducts
400-600 FPM
Larger ducts reduce static pressure
Residential Maximum
900 FPM
Above this = excessive noise
Duct Sizing Formula
Round Duct: Area = CFM ÷ Velocity, Diameter = √(Area ÷ 0.7854) × 12
Rectangular Duct: Area = CFM ÷ Velocity, Dimensions based on aspect ratio
Why velocity matters: Higher velocity = smaller ducts but more noise and pressure drop
Generate Professional Duct Design Reports
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