Oval Duct Size Calculator
Flat oval duct equivalent diameter and size.
Enter your values
Enter the target round equivalent diameter in in.
Enter the minor axis (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.
- Required major axis
- 10 in
- Minor axis (used)
- 6 in
- Approx cross-section area
- 47 sq in
Quick answer
Flat oval duct is described by its major and minor axis, sized to have the same equivalent round diameter as a comparable circular duct.
De = 1.55 x (a x b)^0.625 / (a + b)^0.25 (approx, oval)
At a glance
| What it does | Flat oval duct equivalent diameter and size. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Target round equivalent diameter, Minor axis (duct height) |
| Main output | Required major axis |
| Formula | De = 1.55 x (a x b)^0.625 / (a + b)^0.25 (approx, oval) |
| Cost | Free — no sign-up, no download |
How to use the Oval Duct Size Calculator
- 1Enter the round duct diameter the oval duct needs to match for airflow capacity.
- 2Enter the minor axis (height) available for the flat oval, usually set by ceiling space constraints.
- 3Read off the required major axis (width) to keep equivalent capacity.
Inputs explained
- Target round equivalent diameter(in)
- Enter the target round equivalent diameter in in.
- Minor axis (duct height)(in)
- Enter the minor axis (duct height) in in.
Worked example
Using the values the calculator loads with:
Inputs
- Target round equivalent diameter10 in
- Minor axis (duct height)6 in
Results
- Required major axis10 in
- Minor axis (used)6 in
- Approx cross-section area47 sq in
Frequently asked questions
Why use flat oval duct?
It fits in shallow ceiling or wall cavities where round duct is too tall, while keeping lower friction than rectangular duct.
Is oval duct more expensive?
Typically yes per linear foot versus round or rectangular, due to specialized fabrication.
What do I need to enter into the Oval Duct Size Calculator?
Just 2 values: target round equivalent diameter, minor axis (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 Oval Duct Size Calculator work out the answer?
Flat oval duct is described by its major and minor axis, sized to have the same equivalent round diameter as a comparable circular duct. It applies the formula De = 1.55 x (a x b)^0.625 / (a + b)^0.25 (approx, oval) and shows the working so you can check each step by hand.
Is the Oval Duct Size 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.