Pump Specific Speed Calculator
Specific speed tells you which impeller geometry fits a pump duty — radial, mixed, or axial flow. Enter the operating point and speed to get Ns and the matching impeller type.
Specific Speed
| Item | Value |
|---|
How the calculation works
US customary units (N in rpm, Q in gpm, H in ft)
Ns = N × √Q ÷ H0.75
For double-suction impellers, Q is the flow per side (total Q ÷ 2).
For multistage pumps, H is the head developed per stage.
Ns = N × √Q ÷ H0.75
For double-suction impellers, Q is the flow per side (total Q ÷ 2).
For multistage pumps, H is the head developed per stage.
| Specific speed Ns (US) | Impeller type | Typical duty |
|---|---|---|
| 500 – 4,000 | Radial flow | High head, low to moderate flow |
| 4,000 – 9,000 | Mixed flow | Intermediate head and flow |
| > 9,000 | Axial flow (propeller) | Low head, high flow |
Notes & limitations
Specific speed Ns = N√Q/H0.75 in US units (rpm, gpm, ft) is the standard pump-industry definition (Hydraulic Institute; Karassik’s Pump Handbook). Ranges above are the widely used single-stage, single-suction classification bands.
- Double-suction impellers: the standard definition uses flow per suction side — this tool halves Q when you select double suction.
- Multistage pumps: Ns is computed per stage (total head divided by the number of stages), because each stage is geometrically similar.
- Metric specific speed (Nq) uses different units and is numerically different — do not compare metric and US Ns values directly.
- Ns guides impeller geometry only. Confirm the actual selection against the manufacturer’s published curves, efficiency, and NPSH requirements.
← All water treatment calculators · Related: Pump Horsepower Calculator