What is calculated?
For the same geometry, flow scales linearly, pressure quadratically and power cubically with speed. Small speed increases therefore load the motor strongly.
Equations
V̇₂/V̇₁ = n₂/n₁
Δp₂/Δp₁ = (n₂/n₁)²
P₂/P₁ = (n₂/n₁)³
Fluid dynamics & thermodynamics · preliminary design
Fan Affinity Laws Calculator
Calculate flow, pressure and power of the same fan after a speed change using the fan affinity laws.
IJ 5
02
Results
Enter values and run the calculation.
Application and limits
Assess VFD changes and pre-check motor margin.
Same fan, density and similar curve region; system curve and efficiency can shift the real point.
Frequently asked questions
Why does power scale cubically?
Power is approximately flow times pressure: n·n² = n³.
Does this replace the fan curve?
No. Check the new operating point against fan and system curves.
Technical basis
Dubbel, turbomachinery, similarity/affinity laws and fans (local source, 2026-09-09).
The source supports the equation structure; this calculator does not claim a complete standards verification.