Speed Change — Enter New RPM
Or — Solve for Target CFM → Required RPM
Results — New Fan Conditions
New Static Pressure
—
in wg
Speed vs. Performance Table
| RPM | Speed % | CFM | SP (in wg) | BHP | kW Input | Power vs. Full |
|---|
Fan affinity laws assume the fan operates on the same system curve (duct resistance proportional to flow²). Actual performance may differ due to VFD slip, motor saturation, or system curve changes.
About This Calculator
This calculator applies the fan affinity laws to predict how a fan behaves when its speed changes. Enter the original RPM, CFM, static pressure, and brake horsepower, then enter a new RPM — the tool returns the new airflow, static pressure, BHP, and motor kW input, along with the percent energy savings. You can also solve in reverse: enter a target CFM and it computes the RPM required to reach it.
The affinity laws are the foundation of variable-speed fan control. Because power scales with the cube of speed, even modest speed reductions on a variable-air-volume system yield large energy savings, which is why VFDs dominate modern airside design. The speed-vs-performance table shows the full curve from 50 to 100 percent speed so you can see the trade-off at a glance.
Formula & Method
| Flow | CFM₂ = CFM₁ × (RPM₂ ÷ RPM₁) |
| Static pressure | SP₂ = SP₁ × (RPM₂ ÷ RPM₁)² |
| Brake horsepower | BHP₂ = BHP₁ × (RPM₂ ÷ RPM₁)³ |
| Required RPM | RPM₂ = RPM₁ × (CFMtarget ÷ CFM₁) |
| Motor input | kW = BHP × 0.7457 ÷ ηmotor |
Flow varies linearly with speed, pressure with the square, and power with the cube of the speed ratio, per standard fan-law / ASHRAE practice. The constant 0.7457 converts brake horsepower to kilowatts (1 hp ≈ 0.7457 kW), and dividing by motor efficiency η gives the electrical kW drawn at the motor input. The laws hold while the fan stays on the same system curve, where duct resistance is proportional to flow².
Frequently Asked Questions
What are the fan affinity laws?
The fan affinity laws relate a fan's performance at one speed to its performance at another speed on the same system. Airflow changes in direct proportion to speed (CFM scales with the speed ratio), static pressure changes with the square of the speed ratio, and brake horsepower changes with the cube of the speed ratio. They let you predict new CFM, static pressure, and BHP whenever the fan RPM changes.
Why does fan power drop so much when I slow the fan down?
Because power follows the cube of the speed ratio. Running a fan at 80 percent speed cuts airflow to about 80 percent but cuts shaft power to roughly 0.8 cubed, or about 51 percent. This cubic relationship is the reason variable-frequency drives save so much energy on variable-air-volume systems where the fan rarely needs full flow.
Do the affinity laws always hold exactly?
The affinity laws assume the fan stays on the same system curve, where duct resistance rises with the square of flow. Real systems can deviate due to VFD slip, motor efficiency losses at part load, belt drive losses, or a system curve shifted by dampers or fixed static pressure. Treat the results as a close engineering estimate, not an exact guarantee.
How do I find the RPM needed for a target airflow?
Because airflow is directly proportional to speed, the required RPM equals the original RPM times the ratio of target CFM to original CFM. Enter your target CFM and the calculator solves for the required RPM, then recomputes the resulting static pressure and brake horsepower at that speed.
Results are design estimates for preliminary sizing. Verify final designs against applicable codes and standards — engineering judgment and a licensed professional engineer’s review are required.