Fan Laws Calculator
Change a fan's RPM and see exactly what happens to airflow, pressure and power — the affinity laws that justify variable-speed drives.
Last updated: 2026-09-15
How the calculation works
- Airflow scales linearly with speed — halve the RPM, halve the CFM.
- Pressure scales with the square: slower fans face proportionally less system resistance.
- Power scales with the cube — the famous cubic law that makes 20% slowdown save ~49% energy.
Formula
CFM₂ = CFM₁ × (RPM₂/RPM₁) P₂ = P₁ × (RPM₂/RPM₁)² W₂ = W₁ × (RPM₂/RPM₁)³
| Symbol | Meaning | Unit |
|---|---|---|
n | Rotational speed | RPM |
Q | Airflow | CFM |
P | Static pressure | in. w.c. |
W | Power | W |
Worked example
Slowing a 1,200 RPM fan moving 1,000 CFM at 500 W down to 900 RPM: airflow 750 CFM, pressure ratio 0.56, power 211 W — a 58% energy cut for 25% less air. That's the VFD value proposition.
Interpreting the result
The cubic law means fan speed is the dominant energy lever: precise airflow control via VFD beats damper throttling massively. It also warns the other direction — a belt slipping slower than spec quietly saves energy but under-vents; and overspeeding a fan 20% above spec demands 73% more motor power.
Assumptions
- Same fan, same system, constant air density.
- Operating point stays on the fan's stable curve (no stall).
Limitations
- Static efficiency changes slightly at very low speeds.
- Does not apply across different fan sizes — scaling laws for size differ.
Frequently asked questions
Why does power scale with the cube?
Power = flow × pressure. Flow doubles the mass moved; pressure rises with the square of velocity — multiply the effects and you get the cube.
How much energy does a VFD save?
If it lets you run 20% slower, roughly 49% of fan power — though motor and drive losses shave the real figure to 35–45%.