Altitude Fan Derate Calculator
Fan pressure and horsepower derate at high altitude.
Enter your values
Enter the sea-level rated static pressure in in wc.
Enter the sea-level rated brake hp in hp.
Enter the site altitude in ft.
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.
- Derate factor
- 0.801
- Actual static pressure at altitude
- 2.404 in wc
- Actual brake horsepower at altitude
- 6.01 hp
- Percent reduction
- 19.9%
Quick answer
Fan static pressure and power both derate roughly in proportion to the drop in air density with altitude.
Derate factor = (1 - 6.8753e-6 x altitude)^5.2559
At a glance
| What it does | Fan pressure and horsepower derate at high altitude. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Sea-level rated static pressure, Sea-level rated brake HP, Site altitude |
| Main output | Derate factor |
| Formula | Derate factor = (1 - 6.8753e-6 x altitude)^5.2559 |
| Cost | Free — no sign-up, no download |
How to use the Altitude Fan Derate Calculator
- 1Enter the fan's sea-level rated pressure and horsepower from the catalogue.
- 2Enter the installation altitude.
- 3Read the derate factor and the actual pressure and power the fan produces at that altitude, assuming standard temperature.
Inputs explained
- Sea-level rated static pressure(in wc)
- Enter the sea-level rated static pressure in in wc.
- Sea-level rated brake HP(hp)
- Enter the sea-level rated brake hp in hp.
- Site altitude(ft)
- Enter the site altitude in ft.
Worked example
Using the values the calculator loads with:
Inputs
- Sea-level rated static pressure3.0 in wc
- Sea-level rated brake HP7.5 hp
- Site altitude6000 ft
Results
- Derate factor0.801
- Actual static pressure at altitude2.404 in wc
- Actual brake horsepower at altitude6.01 hp
- Percent reduction19.9%
Frequently asked questions
Does this replace the density correction calculator?
It is the altitude-only portion; combine with a temperature correction if the site is also hot or cold.
Should the motor be oversized for altitude?
Motors themselves lose cooling capacity at altitude too, so check the motor manufacturer's altitude derate separately.
What do I need to enter into the Altitude Fan Derate Calculator?
Just 3 values: sea-level rated static pressure, sea-level rated brake hp, site altitude. 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 Altitude Fan Derate Calculator work out the answer?
Fan static pressure and power both derate roughly in proportion to the drop in air density with altitude. It applies the formula Derate factor = (1 - 6.8753e-6 x altitude)^5.2559 and shows the working so you can check each step by hand.
Is the Altitude Fan Derate 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.