Duct CFM Calculator
How much air a duct can carry at a target velocity.
Enter your values
Choose one of: Round, Rectangular.
Enter the diameter (round) in in.
Enter the width (rect) in in.
Enter the height (rect) in in.
Enter the design velocity in FPM.
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.
- Airflow capacity
- 279 CFM
- Free area
- 0.349 sq ft
- Equivalent round diameter
- 8 in
- In m³/h
- 474
Working
- 10.349 sq ft x 800 FPM = 279 CFM
Quick answer
Duct capacity is the free area multiplied by the design velocity you allow.
CFM = area (sq ft) x design velocity (FPM)
At a glance
| What it does | How much air a duct can carry at a target velocity. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Duct shape, Diameter (round), Width (rect), Height (rect), Design velocity |
| Main output | Airflow capacity |
| Formula | CFM = area (sq ft) x design velocity (FPM) |
| Cost | Free — no sign-up, no download |
How to use the Duct CFM Calculator
- 1Pick the duct shape and enter its inside dimensions.
- 2Set a design velocity — 700–900 FPM for quiet residential branches.
- 3Compare the capacity with the CFM that branch actually needs.
Inputs explained
- Duct shape
- Choose one of: Round, Rectangular.
- Diameter (round)(in)
- Enter the diameter (round) in in.
- Width (rect)(in)
- Enter the width (rect) in in.
- Height (rect)(in)
- Enter the height (rect) in in.
- Design velocity(FPM)
- Enter the design velocity in FPM.
Worked example
Using the values the calculator loads with:
Inputs
- Duct shapeRound
- Diameter (round)8 in
- Width (rect)12 in
- Height (rect)8 in
- Design velocity800 FPM
Results
- Airflow capacity279 CFM
- Free area0.349 sq ft
- Equivalent round diameter8 in
- In m³/h474
- 0.349 sq ft x 800 FPM = 279 CFM
Frequently asked questions
How much CFM does 6 inch duct carry?
About 110–140 CFM at a quiet 600–700 FPM, and up to 175 CFM if you accept more noise.
What is equivalent diameter for?
It lets you compare a rectangular duct against round duct friction charts.
What do I need to enter into the Duct CFM Calculator?
Just 5 values: duct shape, diameter (round), width (rect), height (rect), design velocity. 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 CFM Calculator work out the answer?
Duct capacity is the free area multiplied by the design velocity you allow. It applies the formula CFM = area (sq ft) x design velocity (FPM) and shows the working so you can check each step by hand.
Is the Duct CFM 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.