Round Duct Area Calculator
Cross-sectional area of a round duct.
Enter your values
Enter the duct diameter 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.
- Area
- 50.27 sq in
- In sq ft
- 0.3491
- In m²
- 0.0324
- Circumference
- 25.13 in
Quick answer
Round duct area is pi times the diameter squared divided by four.
A = pi x d² / 4
At a glance
| What it does | Cross-sectional area of a round duct. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Duct diameter |
| Main output | Area |
| Formula | A = pi x d² / 4 |
| Cost | Free — no sign-up, no download |
How to use the Round Duct Area Calculator
- 1Measure the inside diameter of the duct.
- 2Read the area in square feet for airflow maths.
- 3Multiply area by velocity in FPM to get CFM.
Inputs explained
- Duct diameter(in)
- Enter the duct diameter in in.
Worked example
Using the values the calculator loads with:
Inputs
- Duct diameter8 in
Results
- Area50.27 sq in
- In sq ft0.3491
- In m²0.0324
- Circumference25.13 in
Frequently asked questions
Inside or outside diameter?
Use the inside diameter — that is the air path.
Flex duct?
Use the nominal inside diameter when fully stretched; sag reduces it sharply.
What do I need to enter into the Round Duct Area Calculator?
Just 1 value: duct diameter. 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 Round Duct Area Calculator work out the answer?
Round duct area is pi times the diameter squared divided by four. It applies the formula A = pi x d² / 4 and shows the working so you can check each step by hand.
Is the Round Duct Area 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.