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.
Open calculatorLast 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.
Worked example
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.
Fan affinity laws: what changes when speed changes
Percentages of the values at full speed.
| Speed | Airflow | Pressure | Power |
|---|---|---|---|
| 50 % | 50 % | 25 % | 13 % |
| 60 % | 60 % | 36 % | 22 % |
| 70 % | 70 % | 49 % | 34 % |
| 75 % | 75 % | 56 % | 42 % |
| 80 % | 80 % | 64 % | 51 % |
| 90 % | 90 % | 81 % | 73 % |
| 100 % | 100 % | 100 % | 100 % |
Airflow varies with speed, pressure with its square and power with its cube.
Worked scenario: slowing a fan from 1,200 to 900 RPM
At 75 % of the speed the fan moves 750 CFM instead of 1,000, and its power falls from 500 W to 211 W, roughly 42 %. A 25 % reduction in airflow saves close to 58 % of the energy, which is why variable-speed drives pay off on fans.
Conditions for the laws to hold
- The same fan, the same air density and the same system. A change in the ductwork or filters changes the operating point.
- Speed changes of moderate size. Far from the design point, efficiency changes and the laws become approximate.
- Real power savings are slightly smaller than the cube law predicts once motor and drive losses are counted.
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%.