Pump and fan affinity laws calculator

How flow, head and power change with speed.

Calculating…

How it's calculated

Q2 = Q1 × (n2/n1) H2 = H1 × (n2/n1)² P2 = P1 × (n2/n1)³

Because power follows the cube of speed, a 20% speed reduction cuts power by about half. That is why VFDs on fans and pumps often pay back quickly.

The laws apply to centrifugal machines without static head. If a pump must lift against a fixed static head, the savings are smaller, and too low a speed may give no flow at all.