Pump Affinity Law Calculator
The pump affinity laws describe how centrifugal pump performance changes when impeller speed changes. Flow (Q) is directly proportional to speed (N): double the speed, double the...
Formula
Source: Engineering Toolbox, Hydraulic Institute Standards | Last reviewed: June 8, 2026
Examples
0 GPM (US)
= 200 GPM (US)
- Q1 = 100
- N1 = 1750
- N2 = 3500
Doubling speed doubles flow: 100 → 200 gpm
0 GPM (US)
= 250 GPM (US)
- Q1 = 500
- N1 = 3500
- N2 = 1750
Halving speed halves flow: 500 → 250 gpm
0 GPM (US)
= 66.3 GPM (US)
- Q1 = 80
- N1 = 1750
- N2 = 1450
60 Hz to 50 Hz motor (1750 → 1450 rpm)
Where is this used?
Pump retrofitting: calculating new flow when replacing a motor with different speed.
Multi-speed pump applications: determining flow at each speed setting.
Energy savings estimation: reducing speed saves energy per the affinity laws.
Frequently Asked Questions
What are the other affinity laws?
There are three affinity laws: (1) Q ∝ N (flow proportional to speed), (2) H ∝ N² (head proportional to speed squared), (3) P ∝ N³ (power proportional to speed cubed). This calculator handles law #1 for flow. Use squared and cubed relationships for head and power.
Do affinity laws work for positive displacement pumps?
No. Affinity laws apply to centrifugal (rotodynamic) pumps only. Positive displacement pumps have flow directly proportional to speed with a fixed displacement per revolution, regardless of system head.
What limits the range of speed adjustment?
Practical limits include: minimum speed to produce adequate head (below ~30% speed, most centrifugals produce negligible head), maximum speed limited by mechanical design and driver capability, and system curve considerations that may limit the flow range.
Reviewed for accuracy
· Last reviewed: June 8, 2026
All calculations are for reference only. Always verify with manufacturer data and a qualified engineer for critical applications. Learn about our editorial process.