Specific speed (Ns) is a dimensionless-in-spirit indicator of impeller geometry — useful early in pump selection to sanity-check whether a radial, mixed, or axial-flow design fits the duty point, before going to a manufacturer's curves.
Standard method — stated scope limits apply
Duty Point (Best Efficiency Point)
Multistage pumps: enter TOTAL head across all stages — the calculator divides by stage count, since specific speed is a per-stage quantity.
Enter the pump duty point.
About This Calculation
A centrifugal pump's specific speed (Ns) is a dimensionless (or unit-dependent, by convention) number derived from its flow rate, head, and speed at the best efficiency point — it doesn't describe a single pump's performance so much as it classifies which IMPELLER GEOMETRY FAMILY (radial, mixed-flow, axial-flow) is best suited to that duty point, since different geometries are inherently better at different flow/head combinations. This calculator computes Ns for a given duty and identifies the impeller type typically used at that specific speed range, as a pump-selection starting point.
Frequently Asked Questions
Why does specific speed predict impeller GEOMETRY rather than just efficiency?
Different impeller shapes are fundamentally better at converting rotational energy into head versus flow — a narrow, radial-flow impeller excels at high head/low flow duties, while a wide, axial-flow (propeller-type) impeller excels at low head/high flow duties, and specific speed is essentially a dimensionless way of expressing where a given duty point falls on that spectrum.
Can two very different pumps have the same specific speed?
Yes — specific speed is defined per unit of the pump’s BEST EFFICIENCY POINT parameters, so a small pump and a large pump with proportionally similar flow/head/speed relationships (geometrically similar impellers, different size) will have the same specific speed even though their absolute flow and head values differ substantially.
Does a higher specific speed always mean higher pump efficiency?
Not directly — specific speed indicates GEOMETRY FAMILY, and within any family, well-designed pumps of that type can achieve good efficiency; extremely low or extremely high specific speed pumps do tend to have somewhat lower peak efficiency than mid-range values, but this is a secondary effect, not the primary purpose of the number.
Why is specific speed useful before selecting a specific pump model?
It narrows the search to the RIGHT FAMILY of pump for a duty point before comparing specific manufacturer models — trying to force a radial-flow pump design onto a low-head, high-flow duty (or vice versa) generally produces a poorly-performing, inefficient result regardless of how well that specific pump is otherwise built.