Duct Size Calculator (I-P & SI)

The Duct Size Calculator is intended for use as a quick reference tool for approximating duct sizes and equivalent sizes of sheet metal duct versus flexible duct. Includes sizing for metal ducts, and for flexible duct when compressed at 4%, 15%, and 30% straight line compression. Refer to the corresponding field on the wheel to read flexible duct size required to maintain equivalent airflow and same pressure loss for a given metal duct size. One side of the wheel uses Inch-Pound (I-P) units; the other side uses International System (SI) units.

Pressure loss per unit length through a round sheet metal duct or a wire-wound flexible duct was calculated using the Darcy equation. For either type of duct, the friction factor was determined using the Colebrook equation. For more information, please refer to the ASHRAE Handbook--Fundamentals chapter, Duct Design.

The Duct Size Calculator was made possible through collaboration between ASHRAE Technical Committee (TC) 5.2, Duct Design, and the Air Distribution Institute. ASHRAE Research conducted at Tennessee Tech University and Texas A&M University helped provide data used by the Duct Size Calculator

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Citation: ASHRAE Chart