Equivalent Round Duct Calculator
Rectangular ducts are sized through their equivalent round diameter - the round duct with the same friction loss at the same airflow. The working duct size calculator returns the round equivalent and the rectangular options side by side.
Why equivalence
Friction charts and ductulators are built for round duct; a rectangular duct rides the same chart through the diameter that matches its friction per unit length.
The Huebscher relation
The standard conversion (used by ASHRAE friction charts) from width a and height b:
De = 1.30 * (a*b)^0.625 / (a+b)^0.25
Aspect ratio costs
Flat ducts need more metal and more perimeter for the same air: keep aspect ratios modest (<= ~4:1) where the chase allows.
Frequently asked questions
Is equal area the same as equivalent diameter?
No - a rectangle with equal AREA has more perimeter and more friction. The Huebscher equivalent is friction-based and is what the charts expect.
Does the conversion change with airflow?
No; it is pure geometry. The friction at that diameter then scales with airflow per the friction chart.
Related pages
- Duct size calculator - round equivalent and rectangular sizing from CFM and velocity.
- Duct friction loss calculator - the pressure the size decision costs.
- Flex duct capacity chart - the flex penalty at the same sizes.