Equivalent Round Duct Diameter Calculator
Convert rectangular duct to equivalent round.
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ⓘ Live results
Results
- Equivalent round diameter
- 12.19 in
- Rectangular area
- 0.889 sq ft
- Round area at De
- 0.81 sq ft
Quick answer
The ASHRAE equal-friction equivalent round diameter is found from the rectangular duct's two side lengths.
De = 1.30 x (a x b)^0.625 / (a + b)^0.25
How to use the Equivalent Round Duct Diameter Calculator
- 1Measure the two inside dimensions of the rectangular duct.
- 2Apply the ASHRAE equal-friction formula to get the equivalent round diameter.
- 3Use that diameter when comparing to round duct friction charts.
Worked example
Using the values the calculator loads with:
Inputs
- Rectangular side A16 in
- Rectangular side B8 in
Results
- Equivalent round diameter12.19 in
- Rectangular area0.889 sq ft
- Round area at De0.81 sq ft
Frequently asked questions
Why not just match cross-sectional area?
Equal-area conversion does not preserve friction; the ASHRAE formula matches friction rate, which is what design actually needs.
Does aspect ratio matter?
Yes — very flat rectangular ducts need a larger equivalent diameter for the same airflow than square ones.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
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Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.