Equivalent Duct Diameter Calculator
Round equivalent of a rectangular or oval duct.
Enter your values
Enter the duct width in in.
Enter the duct height 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
- 13.48 in
- Rectangular area
- 1.111 sq ft
- Aspect ratio
- 2.5 : 1
- Round duct of equal area
- 14.27 in
Working
- 11.3 x (20 x 8)^0.625 / (28)^0.25 = 13.48 in
Quick answer
Equivalent diameter matches friction loss, not area, and uses the Huebscher relation.
De = 1.3 x (a x b)^0.625 / (a + b)^0.25
At a glance
| What it does | Round equivalent of a rectangular or oval duct. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Duct width, Duct height |
| Main output | Equivalent round diameter |
| Formula | De = 1.3 x (a x b)^0.625 / (a + b)^0.25 |
| Cost | Free — no sign-up, no download |
How to use the Equivalent Duct Diameter Calculator
- 1Enter the inside width and height of the rectangular duct.
- 2Read the equivalent round diameter for friction charts.
- 3Keep the aspect ratio under 4:1 where you can — flatter duct wastes energy.
Inputs explained
- Duct width(in)
- Enter the duct width in in.
- Duct height(in)
- Enter the duct height in in.
Worked example
Using the values the calculator loads with:
Inputs
- Duct width20 in
- Duct height8 in
Results
- Equivalent round diameter13.48 in
- Rectangular area1.111 sq ft
- Aspect ratio2.5 : 1
- Round duct of equal area14.27 in
- 1.3 x (20 x 8)^0.625 / (28)^0.25 = 13.48 in
Frequently asked questions
Why not just match area?
A flat duct has more wetted perimeter, so it rubs more air; equal-friction diameter is always smaller than equal-area diameter.
Does this work for flat oval?
It is a close approximation; for exact flat-oval sizing use the manufacturer's table.
What do I need to enter into the Equivalent Duct Diameter Calculator?
Just 2 values: duct width, duct height. 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 Duct Diameter Calculator work out the answer?
Equivalent diameter matches friction loss, not area, and uses the Huebscher relation. It applies the formula De = 1.3 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 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.