Estimate pump similarity with speed, flow, and head. Choose metric or US units and solve unknowns. Clear results support smarter pump selections.
Pump specific speed compares geometrically similar pumps at similar efficiency. It relates rotational speed, flow rate, and head at a selected duty point.
| # | Speed (rpm) | Flow | Head | Unit System | Specific Speed |
|---|---|---|---|---|---|
| 1 | 1450 | 0.06 m³/s | 18 m | Metric | 33.31 |
| 2 | 1750 | 1200 gpm | 120 ft | US | 2542.12 |
| 3 | 2900 | 0.02 m³/s | 55 m | Metric | 18.44 |
Pump specific speed (Ns or nq) is a similarity index that links a pump’s duty-point speed, flow, and head into one comparable number. It helps engineers judge whether a design behaves like a radial, mixed-flow, or axial-flow machine when operating near best efficiency. Because it is built from the duty-point values, Ns should be evaluated at the same reference point used in selection.
Ns is not a pure “universal constant” unless a consistent definition and unit system are used. The US form typically uses rpm, gpm, and ft, while the metric form uses rpm, m³/s, and m. Switching systems changes the numeric magnitude, so comparisons must be made within the same system.
Low specific speed pumps commonly serve high head, low flow duties such as boiler feed, pressure boosting, and filtration loops. Mid-range values often indicate general centrifugal service in water distribution, HVAC, and process transfer. High values trend toward axial or propeller-like behavior and appear in flood control, cooling water circulation, and large-volume recirculation systems.
As specific speed rises, the impeller passage area typically increases and the flow path becomes more “through-flow” oriented. Radial designs emphasize energy transfer by larger diameter and higher peripheral velocity, while mixed-flow and axial designs emphasize larger inlet area and smoother acceleration of fluid. Ns provides a compact bridge between duty requirements and likely hydraulic shape.
In early design, Ns helps narrow the family of pumps worth reviewing before detailed curve matching. For a required flow and head, changing speed changes Ns and can move a design from radial toward mixed-flow. This is useful when evaluating options such as variable frequency drives, alternative impeller trims, or selecting between two frame sizes.
Use duty-point head and flow from verified test data, vendor curves, or measured operating conditions. If the system curve is uncertain, compute Ns for several plausible duty points to see sensitivity. A small error in head can shift Ns because head is raised to the 3/4 power. Confirm units, especially when converting m³/h, L/min, or ft³/s into the base units used by the chosen system.
The classification labels are a guidance layer, not a standard. Manufacturers may define boundaries differently, and some designs overlap categories. Treat the label as a directional clue about the hydraulic style, expected impeller proportions, and typical efficiency envelopes, then validate using pump curves.
Specific speed does not replace NPSH checks, cavitation margin, efficiency, power, or mechanical constraints. Two pumps can share similar Ns yet differ in materials, clearances, stages, and control strategy. Use this calculator as a screening tool, then complete selection with full performance curves, system requirements, and safety factors for your application.
The index is built from speed, flow, and head, but its numeric value depends on the adopted definition and unit system. Treat it as a comparative index within the same system rather than a universal constant.
Use the intended operating point, ideally near best efficiency on the pump curve. If your system operates across a range, calculate Ns at several points to understand how classification and similarity may change.
The exponent comes from pump similarity scaling relationships connecting head with speed and impeller size. It balances the relationship so pumps with similar hydraulic behavior produce comparable Ns values at comparable efficiency levels.
Not safely. The numbers are on different scales because of the units embedded in the definition. Convert inputs to one system and compute Ns consistently, or compare only within one chosen system.
Accuracy is good if you enter correct values and choose the correct unit. The calculator converts m³/h to m³/s internally for metric calculations. Double-check decimal placement, especially for large flows.
Higher Ns generally aligns with higher flow relative to head, trending toward mixed-flow or axial behavior. These pumps often have larger flow passages and may be more sensitive to inlet conditions and cavitation margin.
No. Ns can correlate with typical efficiency envelopes, but actual efficiency depends on design details, Reynolds number effects, clearances, surface finish, and operating point. Always confirm with manufacturer curves or certified test data.