Fan Affinity Density Calculator
Pressure and power correction for air density change.
Enter your values
ⓘ Live results
Results
- Corrected static pressure
- 1.6 in wc
- Corrected brake horsepower
- 4.8 hp
- Density ratio
- 0.8
- Percent change
- -20%
Quick answer
At a fixed speed and wheel size, CFM stays constant with density but pressure and power scale directly with density ratio.
SP2 = SP1 x (rho2/rho1); BHP2 = BHP1 x (rho2/rho1)
How to use the Fan Affinity Density Calculator
- 1Enter the fan's rated static pressure and horsepower at standard density (0.075 lb/ft3 at sea level, 70F).
- 2Enter the actual air density at your site (lower at altitude or high temperature).
- 3Read the corrected pressure and power the fan actually produces.
Worked example
Using the values the calculator loads with:
Inputs
- Rated static pressure2.0 in wc
- Rated brake HP6 hp
- Rated air density0.075 lb/ft3
- Actual air density0.060 lb/ft3
Results
- Corrected static pressure1.6 in wc
- Corrected brake horsepower4.8 hp
- Density ratio0.8
- Percent change-20%
Frequently asked questions
Why does CFM stay the same?
A fan moves a fixed volume of air per revolution regardless of density; only the mass flow and pressure change.
Where do I get actual density?
Use an air density correction calculator with your site altitude and temperature.
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.