What this calculator does

It sizes a rectangular duct from the airflow by the friction method (for example 1 Pa/m) or the velocity method. You can fix the width or the height. It shows the duct size, velocity, friction rate and equivalent round diameter.

How it is calculated

Friction Darcy–Weisbach with the Colebrook friction factor Air standard air ρ = 1.204 kg/m³, μ = 1.81 × 10⁻⁵ Pa·s Duct galvanised steel, roughness ε = 0.09 mm Rectangle Huebscher equivalent diameter De = 1.30 (a·b)^0.625 ÷ (a + b)^0.25

The method follows ASHRAE Fundamentals duct design. The height is rounded up to 50 mm (2 in).

Questions

What friction rate should I use?

About 0.8–1.0 Pa/m is common for low-pressure supply ducts. Check the velocity too, especially near occupied spaces. The guide Duct sizing by the equal friction method explains it.

How do I get the fan pressure?

Add up the losses of the index run with the Duct ESP Calculator, then size the motor with the Fan Sizing calculator.

Related

Duct ESP Calculator · Fan Sizing · Air Outlet Selection · Duct sizing guide

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Results are for preliminary design. Verify final selections against project specifications, applicable codes and manufacturer data.