Pumps & Rotating Equipment
Pump Affinity Laws Calculator
The pump affinity laws (also called fan laws or similarity laws) predict how flow, head, and power change when the rotational speed of a centrifugal pump changes, assuming the same pump, same impeller diameter, and operation within a similar region of the pump curve. These are approximate scaling relationships.
TypeInteractive calculator — separate from unit conversions
Calculator
rpm
rpm
Result
Speed ratio (N₂/N₁)1.2
Q₂120 m³/h
H₂57.6 m
P₂17.28 kW
Formulas
Flow scaling
Q₂ = Q₁ × (N₂ / N₁)
Head scaling
H₂ = H₁ × (N₂ / N₁)²
Power scaling
P₂ = P₁ × (N₂ / N₁)³
Diagram
Worked example
A pump operates at N₁ = 1000 rpm with Q₁ = 100 m³/h, H₁ = 40 m, P₁ = 10 kW. Speed increases to N₂ = 1200 rpm. What are the new values?
- 01Speed ratio = N₂ / N₁ = 1200 / 1000 = 1.2
- 02Q₂ = Q₁ × 1.2 = 100 × 1.2 = 120 m³/h
- 03H₂ = H₁ × 1.2² = 40 × 1.44 = 57.6 m
- 04P₂ = P₁ × 1.2³ = 10 × 1.728 = 17.28 kW
Result
At 1200 rpm: Q₂ = 120 m³/h, H₂ = 57.6 m, P₂ = 17.28 kW.
FAQ
Is this a pump selection tool?
No. This calculator scales an existing operating point for a speed change. It does not select a pump or compare pump models.
Does this replace pump curves?
No. Pump curves from the manufacturer are the authoritative source for pump performance. Affinity laws provide approximate scaling for speed changes only.
Does this check NPSH?
No. Net Positive Suction Head (NPSH) must be verified separately. The affinity laws do not check for cavitation risk at the new operating condition.
Does this model system curve interaction?
No. The actual operating point is where the pump curve intersects the system curve. This calculator scales the pump curve point without considering system resistance changes.