Specific speed is an index which can be used to characterize the behavior of a pump. Pumps of identical specific speed will, in general, have similar behavior at the rated conditions. It is important that specific speed be calculated for maximum impeller diameter, and at the pump's BEP, considering head per stage. It is possible to use this formula with a trimmed impeller, or at operating conditions away from BEP, but the number will no longer have the same meaning.

The dimensionless form of specific speed is defined as:

More commonly, a dimensional form is used. This equation is listed in both ANSI-HI 14.3.1.3.1-2019ANSI/HI Standard 14.3-2019, American National Standard for Rotodynamic Pumps - for Design and Application, 2019, published by Hydraulic Institute, New Jersey, USA and API 610 11th Edition:ANSI/API 610 11th Edition - Centrifugal Pumps for Petroleum, Petrochemical and Natural Gas Industries, 2010, published by American Petroleum Institute, 1220 L Street, NW, Washington DC, USA

Note: Specific speeds using this definition are often presented as "dimensionless" numbers - but the value returned depends on the unit system used. In such cases it should be made clear what unit system is involved - for example Ns = 4000 (gpm units) or Ns = 77.5 (metric units). There is a conversion factor of 51.6 between the two versions.

• Ns                = Specific speed

• NRated        = Rotational speed in revolutions per minute

• ωRated        = Rotational speed in radians per second

• QRated        = Total pump flow rate in cubic meters per second (US gallons per minute)

• HRated        = head per stage in meters (feet)

There are alternative methods used to calculate specific speed that include dividing the flow rate (Q) in the above equation by 2 if the impeller is of a double suction design. This will result in the calculated specific speed being less by a multiplying factor of , or approximately 0.707 times less. Therefore the user is urged to take caution and determine how the manufacturer of their pump determined specific speed.

Relation to Pump Design

Specific speed is common in pump design - it conveys information about the type of pump or impeller that will be required. Low specific speeds represent radial machines - delivering high head at relatively low flow. High specific speeds represent axial flow devices - delivering high flow at low head. Mixed flow devices have an intermediate specific speed. With specific speed alone it is possible to make an estimate of the physical construction of a pump.