Fan Affinity Density Calculator
Pressure and power correction for air density change.
Enter your values
Enter the rated static pressure in in wc.
Enter the rated brake hp in hp.
Enter the rated air density in lb/ft3.
Enter the actual air density in lb/ft3.
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.
- 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)
At a glance
| What it does | Pressure and power correction for air density change. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Rated static pressure, Rated brake HP, Rated air density, Actual air density |
| Main output | Corrected static pressure |
| Formula | SP2 = SP1 x (rho2/rho1); BHP2 = BHP1 x (rho2/rho1) |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Rated static pressure(in wc)
- Enter the rated static pressure in in wc.
- Rated brake HP(hp)
- Enter the rated brake hp in hp.
- Rated air density(lb/ft3)
- Enter the rated air density in lb/ft3.
- Actual air density(lb/ft3)
- Enter the actual air density in lb/ft3.
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.
What do I need to enter into the Fan Affinity Density Calculator?
Just 4 values: rated static pressure, rated brake hp, rated air density, actual air density. 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 Fan Affinity Density Calculator work out the answer?
At a fixed speed and wheel size, CFM stays constant with density but pressure and power scale directly with density ratio. It applies the formula SP2 = SP1 x (rho2/rho1); BHP2 = BHP1 x (rho2/rho1) and shows the working so you can check each step by hand.
Is the Fan Affinity Density 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.