What does pump specific speed mean?
Specific speed is a design index number used to classify pumps by impeller type and proportion. Formally, it is the speed in rpm at which a geometrically similar pump would deliver one unit of flow against one unit of head. That definition has no practical meaning, so treat specific speed as an index of the pump's hydraulic character.
Two points matter in practice:
- Specific speed is always determined at the best efficiency point (BEP).
- Changing a pump's operating speed does not change its specific speed, because flow and head change together according to the affinity laws.
How do you calculate pump specific speed?
Multiply pump speed by the square root of BEP flow, then divide by BEP head per stage raised to the three quarters power. The number you get depends on the units used, so always state them. The ranges on this page use metric units: flow in cubic meters per second and head in meters.
Ns = N × Q0.5 / H0.75
Where:
- N = pump speed (rpm)
- Q = capacity at BEP (m³/s). For a double suction impeller, use the flow per impeller eye.
- H = total head per stage at BEP (m)
In US practice the same formula is used with Q in gpm and H in feet. A value in US units is about 51.6 times the metric value, so a metric specific speed of 30 is about 1,550 in US units, and 200 is about 10,300.
How does specific speed relate to impeller shape?
Specific speed sets the general shape and class of the impeller. As specific speed rises, the ratio of impeller outlet diameter to inlet diameter (D2:D1) falls, and the flow path turns from radial toward axial. That makes specific speed a quick way to see what type of pump suits a duty.
| Specific speed (metric) | Impeller type | Typical duty | D2:D1 ratio |
|---|---|---|---|
| Under 30 | Radial | Low flow, high head | 2:1 or more |
| Between 30 and 200 | Francis vane and mixed flow | Moderate flow and head | Falls as specific speed rises |
| 200 or more | Axial flow | High flow, low head | Close to 1:1 |
How do different impellers develop head?
The force that generates head shifts from radial to axial as specific speed rises. Low specific speed impellers throw liquid outward and rely mainly on radial (centrifugal) force. High specific speed impellers push liquid along the shaft axis, like a propeller. Impellers in between use both.
- Low specific speed impellers: mainly radial force.
- Francis vane and mixed flow impellers: a combination of radial and axial forces.
- High specific speed impellers: axial forces alone.
Why does specific speed matter when selecting a pump?
Specific speed tells you early in a project whether a duty suits a radial, mixed flow or axial pump, before you look at individual curves. It also hints at curve shape: low specific speed pumps tend to have flatter head curves, while high specific speed pumps have steeper head curves and power curves that change sharply with flow.
Calculate it from your flow, head and a likely motor speed. If the result falls at the edge of a range, a different speed or number of stages may give a better fit. PCT can help match pump type to your duty. Request a quote or see our process pumps.