Duct CFM from Velocity Calculator
Airflow from measured velocity and duct size.
Enter your values
Enter the measured velocity in FPM.
Enter the round duct diameter 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
- 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)
At a glance
| What it does | Airflow from measured velocity and duct size. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Measured velocity, Round duct diameter |
| Main output | Airflow |
| Formula | CFM = FPM x area (sq ft) |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Measured velocity(FPM)
- Enter the measured velocity in FPM.
- Round duct diameter(in)
- Enter the round duct diameter in in.
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.
What do I need to enter into the Duct CFM from Velocity Calculator?
Just 2 values: measured velocity, round duct diameter. 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 from Velocity Calculator work out the answer?
Airflow equals measured velocity in FPM multiplied by the 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 Duct CFM from Velocity 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.