Engineering calculator reviewed for preliminary design use · Last updated: March 2026
Apply pump affinity laws to predict flow rate, head, and shaft power changes from speed or impeller diameter modification. Apply to variable frequency drive analysis and operating condition change evaluation.
Pump Affinity Laws Calculator
The pump affinity laws (similarity laws) predict how a centrifugal pump's performance changes when speed or impeller diameter is changed while keeping the same pump geometry.
Affinity Law Formulas
Flow ratio: Q₂/Q₁ = n₂/n₁ (proportional to speed)
Head ratio: H₂/H₁ = (n₂/n₁)² (proportional to speed squared)
Power ratio: P₂/P₁ = (n₂/n₁)³ (proportional to speed cubed)
Applications
VFD energy savings analysis (50% flow → 12.5% power)
Q₂/Q₁ = N₂/N₁ (flow proportional to speed), H₂/H₁ = (N₂/N₁)² (head proportional to speed squared), P₂/P₁ = (N₂/N₁)³ (power proportional to speed cubed). These apply for geometrically similar conditions.
When do affinity laws not apply?
Affinity laws are approximate. They become less accurate at very low speeds, when operating far from BEP, when system friction dominates, or when cavitation (NPSH) conditions change significantly.
These results are preliminary estimates for screening-level design use. They do not replace detailed engineering, code compliance verification, or vendor-certified calculations.