Engineering Tool — Reference
Fan Affinity Law Calculator
The affinity laws describe how a centrifugal fan behaves when its rotational speed changes. Adjust the speed below to see the effect on airflow, pressure and power relative to the 100% reference point.
Airflow
Q₂/Q₁ = N₂/N₁
80%
Static Pressure
P₂/P₁ = (N₂/N₁)²
64%
Shaft Power
Pw₂/Pw₁ = (N₂/N₁)³
51.2%
Affinity law curves
The affinity laws assume a fixed system curve, constant air density and unchanged fan efficiency. Real systems with static pressure setpoints or filter loading deviate from the ideal cube law — treat the power figure as an upper bound on achievable reduction.
Worked example: 80% speed
Reducing a fan to 80% speed delivers approximately 80% of the airflow, 64% of the static pressure and only 51% of the shaft power. This cube relationship between speed and power is why variable-speed control of fans and pumps is one of the highest-leverage efficiency measures in data center cooling systems.
| Quantity | Law | At 80% |
|---|---|---|
| Flow | Q₂/Q₁ = N₂/N₁ | ≈ 80% |
| Pressure | P₂/P₁ = (N₂/N₁)² | ≈ 64% |
| Power | Pw₂/Pw₁ = (N₂/N₁)³ | ≈ 51% |