Size the duct from airflow and target velocity

A clear, preliminary guide to turning airflow into a round or rectangular duct size, then checking straight-duct pressure loss with the calculator.

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01 / Start with the air, not the duct

Start with the air, not the duct

Duct sizing begins with the volume of air that must move and the velocity you are willing to carry. The required cross-sectional area is the first useful answer; shape and dimensions come after it.

Core sizing relationship

A = Q / V

Use consistent units: area A equals airflow Q divided by target velocity V. For a round duct, the equivalent diameter is D = √(4A/π). For a rectangular duct, choose a practical aspect ratio and preserve the same area.

A transparent preliminary estimate

Air properties use standard air at 20 °C for a first-pass estimate.
The default absolute roughness is 0.09 mm, suitable for a smooth commercial duct assumption.
Straight-duct friction is estimated with Darcy–Weisbach: 64/Re for laminar flow and Haaland for turbulent flow.
The result is preliminary and excludes fittings, transitions, grilles, filters, fans, leakage, and acoustics.

02 / From requirement to dimension

From requirement to dimension

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Enter airflow and choose a target velocity that fits the comfort, noise, and system context.
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Calculate area with A = Q / V, then convert that area into a round diameter or a rectangular pair of sides.
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Check Reynolds number, friction factor, and pressure loss per unit length for the selected dimensions.
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Use the result as a transparent starting point before a full system design accounts for fittings and equipment.

03 / Questions engineers ask first