Enter airflow and a target velocity to get the round diameter (or equivalent rectangular sides). Or enter a size to see the resulting velocity.
m³/h
m/s (4–8 typical for mains)
mm — leave blank for round result
mm — leave blank to solve for it
Required area
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Round diameter
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Rectangular (A×B)
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Velocity check
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Area = Q / v. Round diameter d = √(4A/π). Rectangular: B = A / (A_side/1000) when one side is fixed. Keep velocity in the 4–8 m/s mains band to balance duct cost vs fan power.
How it is sized
Cross-sectional area A = Q / v (with Q in m³/s). A round duct of diameter d carries the same area when A = πd²/4, so d = √(4A/π). For rectangular ducts at equal area, fix one side and solve the other. Pick a size, then run the Duct Loss tool to size the fan for the resulting friction.
Frequently asked questions
What velocity should I size for?
Main supply/return ducts: 4–8 m/s. Branch ducts: 3–5 m/s. Higher velocity saves duct cost but raises friction and fan power.
Round or rectangular?
At equal area round has less surface and lower friction; rectangular is used where ceiling height is tight. This tool gives the equivalent rectangular size.
Does the heat exchanger change the size?
No — sizing is by airflow. But the exchanger adds pressure drop, so size the fan for that too with the Duct Loss tool.
First-order sizing aid. Zibo Qiyu designs full AHU and heat-recovery duct systems with pressure budgets — request a quote.