Pump Affinity Laws Calculator
Slowing a pump with a VFD — or trimming its impeller — changes flow, head, and power in fixed proportions. Enter the rated point and the new speed or diameter to predict the new duty point.
New Duty Point
| Item | Value |
|---|
How the calculation works
Affinity laws (r = N2/N1, or D2/D1 for impeller trim):
Q2 = Q1 · r
H2 = H1 · r²
P2 = P1 · r³
Power falls with the cube of speed — the reason VFDs save so much energy on variable-flow systems.
Q2 = Q1 · r
H2 = H1 · r²
P2 = P1 · r³
Power falls with the cube of speed — the reason VFDs save so much energy on variable-flow systems.
| Speed ratio r | Flow | Head | Power |
|---|---|---|---|
| 1.00 | 100% | 100% | 100% |
| 0.90 | 90% | 81% | 73% |
| 0.80 | 80% | 64% | 51% |
| 0.70 | 70% | 49% | 34% |
| 0.60 | 60% | 36% | 22% |
Notes & limitations
The affinity laws are exact for speed changes at constant efficiency near the best efficiency point (see the Hydraulic Institute standards or any pump handbook). Impeller-trim relations are approximate — efficiency typically drops a few points with a trim.
- Efficiency is assumed roughly constant; far from BEP the prediction drifts. Confirm critical selections against the manufacturer’s published curves.
- System curve matters: the pump will actually operate where its new curve meets the system curve, which this page does not solve — it scales the rated point.
- Do not trim an impeller below the manufacturer’s minimum diameter, and respect minimum-flow limits at reduced speed.
- NPSH required also changes with speed (roughly with the square) — recheck suction conditions after a speed change.
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