Duct Velocity Calculator
Air speed in a duct from CFM and duct size.
Enter your values
Enter the airflow in CFM.
Choose one of: Round, Rectangular.
Enter the round diameter in in.
Enter the rect width in in.
Enter the rect height 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.
- Air velocity
- 1,100 FPM
- Duct area
- 0.545 sq ft
- In m/s
- 5.59
- Noise check
- Typical trunk velocity
Working
- 1600 CFM / 0.545 sq ft = 1,100 FPM
Quick answer
Duct velocity equals airflow in CFM divided by the duct's free area in square feet.
FPM = CFM / area (sq ft)
At a glance
| What it does | Air speed in a duct from CFM and duct size. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Airflow, Duct shape, Round diameter, Rect width, Rect height |
| Main output | Air velocity |
| Formula | FPM = CFM / area (sq ft) |
| Cost | Free — no sign-up, no download |
How to use the Duct Velocity Calculator
- 1Enter the airflow the duct carries in CFM.
- 2Choose the duct shape and enter its inside dimensions.
- 3Keep residential branches under about 900 FPM to stay quiet.
Inputs explained
- Airflow(CFM)
- Enter the airflow in CFM.
- Duct shape
- Choose one of: Round, Rectangular.
- Round diameter(in)
- Enter the round diameter in in.
- Rect width(in)
- Enter the rect width in in.
- Rect height(in)
- Enter the rect height in in.
Worked example
Using the values the calculator loads with:
Inputs
- Airflow600 CFM
- Duct shapeRound
- Round diameter10 in
- Rect width12 in
- Rect height8 in
Results
- Air velocity1,100 FPM
- Duct area0.545 sq ft
- In m/s5.59
- Noise checkTypical trunk velocity
- 600 CFM / 0.545 sq ft = 1,100 FPM
Frequently asked questions
What velocity is too high?
Above roughly 1,500 FPM in residential duct you will hear rush noise at the registers.
Does insulation change it?
Only if it is internal liner — use the inside clear dimensions in that case.
What do I need to enter into the Duct Velocity Calculator?
Just 5 values: airflow, duct shape, round diameter, rect width, rect height. 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 Velocity Calculator work out the answer?
Duct velocity equals airflow in CFM divided by the duct's free area in square feet. It applies the formula FPM = CFM / area (sq ft) and shows the working so you can check each step by hand.
Is the Duct 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.