Calculation
nq = n·√Q / H^0.75
Specific speed is a dimensional index that, independent of size, indicates whether a radial, mixed-flow or axial impeller design suits the best-efficiency point.
Dubbel Strömungsmaschinen R 3.2.1, Gl. (1b): nq = nN·V̇N^(1/2)/HN^(3/4) in min⁻¹ (Bezug Hq = 1 m, V̇q = 1 m³/s)
Specific speed of a centrifugal pump
Specific speed is a dimensional index that, independent of size, indicates whether a radial, mixed-flow or axial impeller design suits the best-efficiency point.
MINTSI
02
Result
Select a target and calculate.
Understand the inputs
- Specific speed nq — Index used to classify impeller type; radial <20, normal 20–40, mixed-flow 40–150, axial >150 (min⁻¹).
- Operating speed n — Pump speed at the considered best-efficiency point.
- Flow rate Q — Volume flow delivered by the pump at the best-efficiency point, per impeller inlet.
- Head H — Head generated by the pump at the best-efficiency point, per stage.
Example
n=1,450 min⁻¹, Q=0.05 m³/s (50 l/s) and H=20 m give nq≈34.3.
Assumptions and limits
Values at the actual best-efficiency point for a single-inlet, single-stage view (for multi-inlet/multi-stage pumps, use Q or H per impeller/stage); the traditional mixed-unit convention (min⁻¹, m³/s, m) is historical and not SI-coherent.
Understand Specific speed of a centrifugal pump
This calculator determines a centrifugal pump's specific speed at the best-efficiency point, the most important figure for preselecting impeller design type in pump development.
What does this quantity describe?
Specific speed combines operating speed, flow rate and head at the best-efficiency point into a size-independent index: nq=n·√Q/H^0.75. Traditionally n is in rev/min, Q in m³/s and H in m.
Formula and variables
nq = n·√Q / H^0.75
nq = n·√Q / H^0.75
| Symbol / input | Meaning |
|---|---|
| Specific speed nq | Index used to classify impeller type; radial <20, normal 20–40, mixed-flow 40–150, axial >150 (min⁻¹). |
| Operating speed n | Pump speed at the considered best-efficiency point. |
| Flow rate Q | Volume flow delivered by the pump at the best-efficiency point, per impeller inlet. |
| Head H | Head generated by the pump at the best-efficiency point, per stage. |
Choose the inputs correctly
n is operating speed at the best-efficiency point, Q the flow rate per impeller inlet and H the head per stage at the same point.
How to use the calculator
Read n, Q and H from the pump performance curve at the best-efficiency point; for multi-inlet impellers divide Q by the number of inlets, for multi-stage pumps divide H by the number of stages.
Worked example
n=1,450 min⁻¹, Q=0.05 m³/s (50 l/s) and H=20 m give nq≈34.3.
Understand the result and units
Small nq values point to radial impellers for high head at low flow; large nq values point to axial impellers for high flow at low head.
n is a rotational speed, Q a volume flow, H a length (head). nq is output as a dimensionless numerical value, historically interpreted in rev/min.
Typical applications
Preselecting pump design type at the concept stage, estimating achievable efficiency, and comparing pumps independent of size.
Assumptions, limits and common mistakes
Values at the actual best-efficiency point are assumed; for multi-inlet or multi-stage pumps, Q or H must already be correctly referenced to one impeller or stage. The traditional mixed-unit convention is not SI-coherent.
Common mistake: Do not enter the total flow of a multi-inlet pump without dividing it across the individual impeller inlets.
Frequently asked questions
What is “Specific speed of a centrifugal pump” used for?
Preselecting pump design type at the concept stage, estimating achievable efficiency, and comparing pumps independent of size.
Where do the input values come from?
n is operating speed at the best-efficiency point, Q the flow rate per impeller inlet and H the head per stage at the same point.
What does the result not cover?
Values at the actual best-efficiency point are assumed; for multi-inlet or multi-stage pumps, Q or H must already be correctly referenced to one impeller or stage. The traditional mixed-unit convention is not SI-coherent.