Duct CFM from Velocity Calculator
Airflow from measured velocity and duct size.
Enter your values
ⓘ Live results
Results
- Airflow
- 464 CFM
- Duct area
- 0.545 sq ft
- In m³/h
- 788
- In L/s
- 219
Working
- 1850 FPM x 0.545 sq ft = 464 CFM
Quick answer
Airflow equals measured velocity in FPM multiplied by the duct area in square feet.
CFM = FPM x area (sq ft)
How to use the Duct CFM from Velocity Calculator
- 1Take a traverse of velocity readings across the duct and average them.
- 2Enter the average FPM and the duct diameter.
- 3Compare the CFM against the design airflow for that branch.
Worked example
Using the values the calculator loads with:
Inputs
- Measured velocity850 FPM
- Round duct diameter10 in
Results
- Airflow464 CFM
- Duct area0.545 sq ft
- In m³/h788
- In L/s219
- 850 FPM x 0.545 sq ft = 464 CFM
Frequently asked questions
Why traverse?
Velocity is highest at the centre; a single centre reading overstates airflow by 10-20%.
Hot-wire or vane?
Hot-wire suits low velocities in ducts; vane meters suit grille face readings.
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.