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Pump Specific Speed Calculator

Enter pump RPM, flow and head to find the specific speed.

How it is calculated

The tool takes the speed N in rpm, the flow Q in US gallons per minute and the head H in feet, and computes the US specific speed Ns = N × √Q ÷ H0.75, rounded to a whole number. The head is the head per stage, so for a multistage pump divide the total head by the number of stages first. It also gives the metric form nq = N × √Q ÷ H0.75, with Q converted to m³/s (× 0.0000630902) and H to metres (× 0.3048); this value is shown to two decimals. The two numbers differ by a fixed factor of about 51.6, so they carry the same information. All three inputs must be above zero. Evaluate the figures at the best-efficiency point of the pump, not at an arbitrary operating point.

Worked example

A pump runs at 1,750 rpm, delivering 500 gpm against 100 ft of head per stage. The square root of 500 is 22.36 and 100 raised to the power 0.75 is 31.62. So Ns = 1,750 × 22.36 ÷ 31.62 = 1,237 (the tool shows 1,237). In metric units the flow is 0.03154 m³/s and the head is 30.48 m, which gives nq = 23.96. If the same duty were run at 3,500 rpm, both numbers would double, to 2,475 and 47.92, because specific speed is proportional to rotational speed.

Try this example →

Common mistakes

  • Entering the total head of a multistage pump. The tool wants head per stage; use the total divided by the number of stages, otherwise the specific speed is too low.
  • Using the wrong units, such as litres per minute or metres for the head. The inputs are US gallons per minute and feet.
  • Comparing a US Ns with a metric nq value from a catalogue. They differ by a factor of about 51.6, so only compare figures in the same system.
  • Using flow and head from a random operating point. The classification refers to the best-efficiency point.
  • Expecting the tool to pick the impeller type. It gives the index; matching it to radial, mixed-flow or axial designs is for you or the pump manufacturer.

About this calculator

Specific speed is a dimensionless-style index, N_s = N√Q ÷ H^0.75 in US units (N in rpm, Q in gpm, H in feet) or n_q in the equivalent metric form, that describes the shape of impeller a pump needs to deliver a given flow (Q) and head (H) at a given rotational speed (N) — independent of the pump's actual physical size.

Low specific speed values correspond to radial-flow impellers suited to high head and low flow, mid-range values to mixed-flow designs, and high values to axial-flow (propeller-type) impellers suited to high flow and low head. Mechanical and process engineers compute specific speed early in pump selection to identify the right impeller family for an application and to flag designs likely to suffer cavitation or efficiency problems before a pump is ever purchased.

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