Understanding Equivalent Duct Size
In HVAC design, round ducts are the most efficient for airflow. However, due to space constraints in ceilings or walls, rectangular ducts are often used. An "equivalent" rectangular duct is one that will have the same friction loss per foot as a round duct when carrying the same amount of air.
This calculator uses the Jones Equation, a common industry standard, to find the equivalent dimensions. The goal is to maintain the same airflow characteristics while changing the shape of the duct to fit the available space.
How to Use the Calculator
- Enter the diameter of the round duct you need to replace.
- Enter the known dimension of one side of your rectangular duct (e.g., the height available in a ceiling joist bay).
- Click "Find Equivalent Size" to calculate the required dimension of the other side of the rectangular duct.