If you have round ducts, you simply measure the diameter (D) of the duct (the cross-section). This will tell you what size duct to purchase to replace it. If you have rectangular or square ducts, measure the width (W) and height (H) of the duct and use these figures when buying new parts.

How do you measure duct size?

Measure the end of a square or rectangular duct each way to determine the size. If the duct fitting inside the wall transitions to a circular duct, measure inside the circular duct from top to bottom. Repeat this at the remaining grills and ducts throughout the house.

What is duct diameter?

The equivalent diameter is the diameter of a circular duct or pipe that gives the same pressure loss as an equivalent rectangular duct or pipe. … The rectangular dimensions and the air flow volume are adapted to the equal friction loss method of sizing ventilation duct systems.