FPM to CFM Calculator
Airflow from measured velocity and duct area.
Enter your values
ⓘ Live results
Results
- 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)
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.
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.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.