Process & Mechanical
Fan / Blower Laws & Power
Fan affinity laws for a speed change (constant air density), and shaft power from flow, pressure rise and total efficiency.
Standard method — stated scope limits apply
Affinity Laws (Speed Change)
Shaft Power
Enter affinity law or power inputs.
About This Calculation
Centrifugal and axial fans follow predictable affinity (fan) laws when their speed changes: flow scales directly with speed, pressure (head) scales with speed squared, and power scales with speed cubed — which is exactly why variable-speed fan drives save so much energy at reduced flow: a fan running at 80% speed uses only about half the power of running at 100% speed, even though flow only dropped 20%. This calculator applies the affinity laws for a speed change and separately calculates fan shaft power from flow, pressure rise, and efficiency.
Frequently Asked Questions
Why does fan power scale with speed CUBED, not just linearly with flow?
Power is the product of flow and pressure, and since flow scales with speed while pressure scales with speed squared, their product scales with speed cubed — this compounding relationship is exactly why a modest fan speed reduction (via a VFD, for instance) produces a disproportionately large energy saving.
Do the fan affinity laws apply to a completely different fan, or only the same fan at different speeds?
Strictly, they apply to the SAME fan (or geometrically similar fans of different size) at different speeds — comparing two genuinely different fan designs at the same speed doesn’t follow the affinity laws, since those laws describe how one fixed geometry’s performance scales, not how different geometries compare to each other.
Why does changing fan speed also change the system operating point, not just the fan curve?
The fan’s performance curve shifts according to the affinity laws, but the SYSTEM resistance curve (duct/damper friction) doesn’t change with fan speed — the new operating point is wherever the shifted fan curve intersects the UNCHANGED system curve, which is why a simple percentage speed change doesn’t produce the exact same percentage flow change if the system curve isn’t purely fan-static-pressure-dominated.
How is fan shaft power actually calculated from flow and pressure?
Shaft power equals flow rate times pressure rise, divided by the fan’s efficiency at that operating point — the efficiency term matters because a fan’s efficiency itself varies across its operating range, so shaft power isn’t simply proportional to the useful (flow × pressure) work being done on the air.