Pump Affinity Laws Calculator
Enter a reference flow, head, and power with their own unit labels, then compare 1–20 speed-ratio scenarios. The browser applies Q₂ = Q₁r, H₂ = H₁r², and P₂ = P₁r³.
A QUICK WALKTHROUGH
How to use this tool
- Enter nonnegative reference flow, head, and shaft power, plus a unit label for each value.
- Enter one or more speed ratios r = N₂/N₁, one per line, and optionally describe the operating conditions.
- Calculate the estimated values, review every scenario, then copy or download the immutable text report.
Affinity-law estimate
For the same rotodynamic pump geometry, this calculator estimates Q₂ = Q₁r, H₂ = H₁r², and P₂ = P₁r³, where r = N₂/N₁. Use a consistent unit for each reference quantity and supply its label; the calculator does not convert units.
Operating-point limitation
These relations are approximate and are most useful for centrifugal-pump applications with little or no static head. A system with static head or a materially changing system curve needs a pump curve and system curve to find the actual new operating point. The affinity-law power estimate assumes efficiency remains approximately unchanged across the speed change. Actual efficiency can vary; the calculator does not predict actual efficiency, motor input power, cavitation, or equipment limits.
Source
U.S. Department of Energy, Adjustable Speed Pumping Applications, Pumping Systems Tip Sheet #11 (2007), describes the affinity equations and warns that systems with static head require a system curve: https://www1.eere.energy.gov/manufacturing/tech_assistance/pdfs/38947.pdf
GOOD TO KNOW
Common questions
What does the ratio r mean?
r is the new rotational speed divided by the reference rotational speed, N₂/N₁. Enter a finite nonnegative ratio; 1 means unchanged speed.
Can I use any pump type?
Treat results as approximate for the same rotodynamic pump geometry. The equations are commonly used for centrifugal pumps; they do not model positive-displacement pump behavior.
Does this find the actual duty point in a static-head system?
No. Static head changes the system curve intersection. Use pump and system curves to determine the actual operating point.
Are units converted?
No. Enter a unit label for each reference value. Each estimated result keeps the matching reference unit.