FPM to CFM Calculator
Airflow from measured velocity and duct area.
Enter your values
Choose one of: Round, Rectangular.
Enter the measured velocity in FPM.
Enter the diameter (round) in in.
Enter the width (rect) in in.
Enter the height (rect) 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.
- Airflow
- 491 CFM
- Duct area
- 0.545 sq ft
- In m³/h
- 834
- In L/s
- 231.7
Working
- 1900 FPM x 0.545 sq ft = 491 CFM
Quick answer
CFM equals velocity in feet per minute multiplied by duct area in square feet.
CFM = FPM x area (sq ft)
At a glance
| What it does | Airflow from measured velocity and duct area. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Duct shape, Measured velocity, Diameter (round), Width (rect), Height (rect) |
| Main output | Airflow |
| Formula | CFM = FPM x area (sq ft) |
| Cost | Free — no sign-up, no download |
How to use the FPM to CFM Calculator
- 1Take an average velocity reading across the duct, not a single centre point.
- 2Enter the duct's internal dimensions.
- 3Read the airflow, then compare it with the design CFM for that branch.
Inputs explained
- Duct shape
- Choose one of: Round, Rectangular.
- Measured velocity(FPM)
- Enter the measured velocity in FPM.
- 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.
Worked example
Using the values the calculator loads with:
Inputs
- Duct shapeRound
- Measured velocity900 FPM
- Diameter (round)10 in
- Width (rect)14 in
- Height (rect)8 in
Results
- Airflow491 CFM
- Duct area0.545 sq ft
- In m³/h834
- In L/s231.7
- 900 FPM x 0.545 sq ft = 491 CFM
Frequently asked questions
How do I average the reading?
Use a traverse: take readings on a grid of equal-area points and average them.
Is centre velocity accurate?
Centre velocity runs high; multiply by about 0.9 for a rough duct average if you cannot traverse.
What do I need to enter into the FPM to CFM Calculator?
Just 5 values: duct shape, measured velocity, diameter (round), width (rect), height (rect). 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 FPM to CFM Calculator work out the answer?
CFM equals velocity in feet per minute multiplied by duct area in square feet. It applies the formula CFM = FPM x area (sq ft) and shows the working so you can check each step by hand.
Is the FPM to 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.