Altitude Fan Derate Calculator
Fan pressure and horsepower derate at high altitude.
Enter your values
ⓘ Live results
Results
- 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
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.
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.
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.