Equivalent Round Duct Diameter Calculator
Convert rectangular duct to equivalent round.
Enter your values
Enter the rectangular side a in in.
Enter the rectangular side b in in.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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
At a glance
| What it does | Convert rectangular duct to equivalent round. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Rectangular side A, Rectangular side B |
| Main output | Equivalent round diameter |
| Formula | De = 1.30 x (a x b)^0.625 / (a + b)^0.25 |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Rectangular side A(in)
- Enter the rectangular side a in in.
- Rectangular side B(in)
- Enter the rectangular side b in in.
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.
What do I need to enter into the Equivalent Round Duct Diameter Calculator?
Just 2 values: rectangular side a, rectangular side b. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Equivalent Round Duct Diameter Calculator work out the answer?
The ASHRAE equal-friction equivalent round diameter is found from the rectangular duct's two side lengths. It applies the formula De = 1.30 x (a x b)^0.625 / (a + b)^0.25 and shows the working so you can check each step by hand.
Is the Equivalent Round Duct Diameter Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.