Velocity Pressure to FPM Calculator
Convert pitot velocity pressure to air speed.
Enter your values
Enter the velocity pressure in in wc.
Enter the 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.
- Air velocity
- 896 FPM
- In m/s
- 4.55
- Airflow
- 488 CFM
- Duct area
- 0.545 sq ft
Working
- 14005 x sqrt(0.05) = 896 FPM
Quick answer
Air velocity equals 4,005 times the square root of velocity pressure in inches of water at standard air density.
FPM = 4005 x sqrt(Vp)
At a glance
| What it does | Convert pitot velocity pressure to air speed. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Velocity pressure, Duct diameter |
| Main output | Air velocity |
| Formula | FPM = 4005 x sqrt(Vp) |
| Cost | Free — no sign-up, no download |
How to use the Velocity Pressure to FPM Calculator
- 1Take the velocity pressure reading with a pitot tube and manometer.
- 2Average a proper traverse of readings across the duct.
- 3Multiply the average velocity by the duct area for airflow.
Inputs explained
- Velocity pressure(in wc)
- Enter the velocity pressure in in wc.
- Duct diameter(in)
- Enter the duct diameter in in.
Worked example
Using the values the calculator loads with:
Inputs
- Velocity pressure0.05 in wc
- Duct diameter10 in
Results
- Air velocity896 FPM
- In m/s4.55
- Airflow488 CFM
- Duct area0.545 sq ft
- 4005 x sqrt(0.05) = 896 FPM
Frequently asked questions
Where does 4005 come from?
It bundles standard air density of 0.075 lb/ft³; correct it for hot or high-altitude air.
Total, static and velocity pressure?
Total equals static plus velocity; a pitot tube measures the difference to give velocity pressure directly.
What do I need to enter into the Velocity Pressure to FPM Calculator?
Just 2 values: velocity pressure, 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 Velocity Pressure to FPM Calculator work out the answer?
Air velocity equals 4,005 times the square root of velocity pressure in inches of water at standard air density. It applies the formula FPM = 4005 x sqrt(Vp) and shows the working so you can check each step by hand.
Is the Velocity Pressure to FPM 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.