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Pumps & Rotating Equipment

Pump Specific Speed Calculator

Pump specific speed is a dimensional index that characterises an impeller by tying together its rotational speed, flow, and head into a single number. It is used early in sizing as a classifier and sanity check: roughly what impeller type suits the duty (radial, mixed-flow, or axial), and whether the speed/flow/head combination is self-consistent. This calculator uses the metric (SI-rpm) convention, stated explicitly: Ns = n·√Q / H^(¾), with n in rpm, Q in m³/s, and H the head per stage in metres. Flow is entered in m³/h and converted to m³/s internally, and head per stage is the total head divided by the number of stages. Specific speed is convention-sensitive — the metric value here is not the same number as the US-unit (gpm-ft-rpm) form, so values must never be compared across conventions. It is a preliminary classifier only: not pump selection, not hydraulic design, and not a substitute for vendor curves. It is most meaningful when evaluated at the pump best-efficiency point (BEP), not at an arbitrary operating point.

TypeInteractive engineering calculator

Calculator

Convention: Ns = n·√Q / H^(¾) — metric (SI-rpm) form with n in rpm, Q in m³/s, and H the head per stage in metres. This is a different number from the US (gpm-ft-rpm) convention — never compare values across conventions. Specific speed is defined at the best-efficiency point (BEP).

Duty point (use BEP flow where available)
rpm
m³/h

Converted to m³/s internally (÷3600)

m

Head at the duty point

Head per stage = total head ÷ stages

Flow rate0.0277778 m³/s
Head per stage50 m
Specific speed (metric Ns)12.8525

Preliminary band

Low specific speed

Likely impeller type: Radial-flow centrifugal

High head, low flow per stage — radial impellers. Very low values may indicate a duty better served by a multistage or positive-displacement pump; re-check the duty and stage count.

Preliminary classifier only. Not pump selection, not hydraulic design, and not a substitute for vendor curves. Specific speed is convention-sensitive and best evaluated near the BEP. Confirm impeller type and selection with vendor data and a qualified engineer.

Related: Pump Sizing · Pump Affinity Laws · Total Dynamic Head · Pump Power

Formulas

Specific speed (metric)
Ns = n · √Q / H^(3/4)
Flow conversion
Q [m³/s] = Q [m³/h] / 3600
Head per stage
H = H_total / number of stages

Diagram

specific speed Ns (metric) — increasing →radiallow Nsmixed-flowmedium Nsaxialhigh Ns

Worked example

A single-stage pump runs at 1450 rpm and delivers 100 m³/h at 50 m head near its best-efficiency point. Calculate the metric specific speed.

  1. 01Q = 100 m³/h ÷ 3600 = 0.02778 m³/s
  2. 02√Q = √0.02778 = 0.1667
  3. 03H^(3/4) = 50^0.75 ≈ 18.80
  4. 04Ns = 1450 × 0.1667 / 18.80
  5. 05Ns ≈ 12.86
Result

Ns ≈ 12.86 (metric convention) — a low-to-medium specific speed, consistent with a radial-flow centrifugal impeller. Use only as a preliminary classifier; confirm against vendor data.

FAQ

What is pump specific speed used for?
It is a preliminary classifier: a single index tying speed, flow, and head together that indicates the broad impeller type (radial, mixed-flow, or axial) suited to a duty, and that flags when a speed/flow/head combination is inconsistent. It is a sanity check early in sizing, not a selection or design tool.
Which specific-speed convention does this use?
The metric (SI-rpm) convention: Ns = n·√Q/H^¾ with n in rpm, Q in m³/s, and H the head per stage in metres. This is disclosed because specific speed is convention-sensitive — the metric value is a different number from the US-unit (gpm-ft-rpm) form for the same pump.
Can I compare this value with a US specific speed?
No. The metric Ns and the US Ns are computed with different units and are numerically different for the same pump. Never compare specific-speed values unless the formula and unit convention match. Convert deliberately, or compute both in the same convention.
Why should I use the BEP flow?
Specific speed is defined at the best-efficiency point. Using an off-BEP flow gives a number that does not correspond to the impeller classification, so the value loses its meaning as a classifier. Enter the BEP flow and the head at BEP where they are available.
How do stages affect specific speed?
Specific speed is based on the head per stage, not the total head, because each stage develops a share of the head. Divide the total head by the number of stages (this calculator does that for you), so a multistage pump is classified by what one impeller does.
Is specific speed enough to choose a pump?
No. It is a preliminary classifier only. Final pump selection requires the vendor pump curve, NPSHr, efficiency and materials for the service, operating cases, and qualified engineering review. Treat the impeller-type band here as indicative context, not a decision.

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