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HVAC Air Duct Sizing Calculator

Calculate round duct diameter and rectangular size for airflow CFM.

Tool inputs stay in your browser · optional AI search sends your query

Good to know: Calculates duct cross-sectional area based on CFM volume and recommended HVAC airflow velocity.

How to use it (step by step)

  1. 1Fill in the boxes: “Airflow Volume (CFM)”, “Air Velocity (Feet per Minute - FPM)”. Example numbers are already there so you can see how it works; replace them with yours.
  2. 2The result updates instantly as you type (on a phone it is below the boxes).
  3. 3The bold line is the main answer. Tap Copy next to any value to copy it.

Words to know

Area / volume / perimeter
Perimeter = distance around a shape. Area = the flat space inside it (m²). Volume = the space inside a 3D object (m³).

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