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🌀 Pump Specific Speed (Ns) Calculator

Calculate the pump specific speed to determine the optimal impeller type (Radial, Mixed, or Axial).

Inputs

RPM

Results

Specific Speed (N_s)0
Recommended Impeller

Impeller Selection Chart

Specific Speed (N_s)RadialMixedAxial

Calculator Description

Specific speed (Ns) is a dimensionless design parameter that characterizes the shape of a pump, especially its impeller. Two pumps that deliver the same head and flow may have radically different impeller geometries — radial, mixed, or axial — and that geometry is captured by specific speed. In short, Ns represents the "skeleton" of the impeller.

What this calculator finds

This calculator takes the rotational speed N, flow rate Q and head H at the best efficiency point (BEP) and computes the pump specific speed Ns. From this value you can select the proper impeller type and assess efficiency, suction performance and the operating range.

Why it matters

  • Quickly deciding the impeller form (radial/mixed/axial) at the design stage
  • Predicting the most efficient pump type from the efficiency-vs-Ns relationship
  • Estimating cavitation risk and net positive suction head (NPSH) margin
  • Scaling and performance translation between geometrically similar pump models

Formula

Pump Specific Speed Equation

Specific speed is defined as the speed at which a geometrically similar impeller would deliver unit head at unit flow; in practice it is evaluated at the best efficiency point.

Ns=N Q0.5H0.75N_s = \dfrac{N\,Q^{0.5}}{H^{0.75}}
  • Ns — Specific speed (dimensionless; numerical range depends on the unit system)
  • N — Pump speed [RPM]
  • Q — Flow rate at best efficiency point [m³/h or US gpm]
  • H — Head per stage at best efficiency point [m or ft]

How the formula works

  • A larger flow Q raises Ns → high-flow (axial) impeller.
  • A larger head H lowers Ns → high-head (radial) impeller.
  • A higher speed N raises Ns, allowing a smaller, high-speed pump for the same duty.

Impeller Selection Guide

  • Radial Flow: Low flow, high head. Ns < 4000 (US)
  • Mixed Flow: Medium flow, medium head. Ns = 4000 ~ 8000 (US)
  • Axial Flow: High flow, low head. Ns > 8000 (US)

Worked example

Consider a pump running at 1750 RPM, BEP flow 100 m³/h and per-stage head 50 m. With Q^0.5 ≈ 10 and H^0.75 ≈ 18.8, Ns ≈ 1750 × 10 / 18.8 ≈ 930 (metric basis), placing it in the radial-flow region.

Useful Tips

Practical tips

  • For multi-stage pumps, use the head per stage (H/stages) to get a correct Ns.
  • Turbine specific speed uses the same form, with flow referred to unit head.
  • The same pump yields different Ns numbers under US vs metric units; always compare on the same basis.

Limitations & cautions

  • Ns is a shape-comparison index, not a performance guarantee; real efficiency depends on design and manufacturing.
  • Very high or very low Ns narrows the operating range and reduces cavitation margin.
  • For viscous fluids, Ns alone is insufficient and correction is required.