Duct Cross-Section Area Calculator
Cross-sectional area of round or rectangular duct.
Enter your values
Choose one of: Round, Rectangular.
Enter the diameter or width in in.
Enter the height (rectangular only) in in.
Enter the airflow in CFM.
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.
- Cross-section area
- 78.5 sq in (0.545 sq ft)
- Resulting velocity
- 917 FPM
Quick answer
Duct cross-section area determines velocity for a given CFM: round ducts use pi/4 x diameter squared, rectangular ducts use width x height.
Area (round) = pi/4 x D^2; Area (rect) = W x H
At a glance
| What it does | Cross-sectional area of round or rectangular duct. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Duct shape, Diameter or width, Height (rectangular only), Airflow |
| Main output | Cross-section area |
| Formula | Area (round) = pi/4 x D^2; Area (rect) = W x H |
| Cost | Free — no sign-up, no download |
How to use the Duct Cross-Section Area Calculator
- 1Choose round or rectangular duct shape.
- 2Enter the diameter, or width and height for rectangular.
- 3Enter airflow to see the resulting velocity through that cross-section.
Inputs explained
- Duct shape
- Choose one of: Round, Rectangular.
- Diameter or width(in)
- Enter the diameter or width in in.
- Height (rectangular only)(in)
- Enter the height (rectangular only) in in.
- Airflow(CFM)
- Enter the airflow in CFM.
Worked example
Using the values the calculator loads with:
Inputs
- Duct shapeRound
- Diameter or width10 in
- Height (rectangular only)8 in
- Airflow500 CFM
Results
- Cross-section area78.5 sq in (0.545 sq ft)
- Resulting velocity917 FPM
Frequently asked questions
Why does velocity matter here?
The same duct area can be too fast or too slow depending on airflow, so always check velocity against comfort and noise targets.
Which is more efficient, round or rectangular?
Round duct has less surface area per unit of airflow, so it is generally more efficient and cheaper per CFM carried.
What do I need to enter into the Duct Cross-Section Area Calculator?
Just 4 values: duct shape, diameter or width, height (rectangular only), airflow. 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 Duct Cross-Section Area Calculator work out the answer?
Duct cross-section area determines velocity for a given CFM: round ducts use pi/4 x diameter squared, rectangular ducts use width x height. It applies the formula Area (round) = pi/4 x D^2; Area (rect) = W x H and shows the working so you can check each step by hand.
Is the Duct Cross-Section 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.