Specific speed n_q calculator
Determine the specific speed n_q of a centrifugal pump from speed, flow rate and head at the best efficiency point. The number classifies the impeller by type – from the slow radial impeller through the mixed-flow impeller to the axial propeller. Number of stages and suction flows are accounted for, and the US specific speed N_s is reported as well.
n_q calculator (impeller type)
Model: specific speed n_q = n·√Q/H^0.75 (European definition, dimensional). Values per impeller and per stage. The type limits are rough guide values; NPSH, efficiency and cavitation are not covered by this number.
Results
Calculating …
Formulas and fundamentals
The specific speed is n_q = n · √Q / H^0.75 with the speed n in rpm, the flow rate Q in m³/s and the head H in m. n_q is a dimensional similarity number: it characterises the model pump that delivers geometrically similar flow at H = 1 m and Q = 1 m³/s. The values per impeller and per stage always govern the classification – not the overall machine values.
For multistage pumps the head is shared among the stages: H/number of stages enters the formula. For double-suction impellers the flow splits between two impeller halves: Q/number of flows enters the formula. Both measures raise n_q compared with the overall values and shift the type towards higher-speed impellers.
By n_q the rough type limits are: n_q < 25 marks the radial impeller (slow runner, high head, small flow), 25 to 60 the mixed-flow impeller and from about 60 (up to roughly 200) the axial propeller impeller (large flow, small head). The English-language number N_s (rpm, gpm, ft) follows from N_s ≈ 51.64 · n_q.
Worked example
A single-stage, single-suction centrifugal pump runs at n = 1450 rpm and delivers Q = 50 m³/h at H = 30 m at the best efficiency point. For the formula the flow rate is converted to m³/s: 50 m³/h / 3600 = 0.013889 m³/s.
This gives n_q = 1450 · √0.013889 / 30^0.75 = 1450 · 0.11785 / 12.82 ≈ 13.3. The specific speed is well below 25.
The impeller is therefore a radial impeller (slow runner): typical of comparatively high head at moderate flow. The US specific speed is N_s ≈ 51.64 · 13.3 ≈ 688.
Frequently asked questions
What does the specific speed n_q mean?
n_q is a similarity number formed solely from speed, flow rate and head at the best efficiency point. It characterises the impeller shape independently of the absolute size and allows classification into radial, mixed-flow or axial impellers.
In which units is n_q calculated?
In the European definition with n in rpm, Q in m³/s and H in m. n_q is dimensional, so the numeric value depends on the unit system. The calculator accepts the flow rate in m³/h if preferred and converts internally to m³/s.
How do stages and double-suction impellers act?
The values per impeller and per stage govern. For multistage pumps H is divided by the number of stages, for double-suction impellers Q by the number of flows (2). Both raise n_q compared with the overall machine values.
Which type belongs to which n_q?
Rough limits: n_q < 25 radial impeller (slow runner), 25 to 60 mixed-flow impeller, from about 60 up to roughly 200 axial impeller (propeller). The limits are gradual and serve for orientation, not as a sharp class boundary.
What is the US specific speed N_s?
N_s is the specific speed common in the Anglo-American world with n in rpm, Q in US gallons per minute and H in feet. It relates to the European number via N_s ≈ 51.64 · n_q and yields considerably larger numeric values.
Related tools
Pump Power & Head
Calculate system head, hydraulic power and shaft and motor power of centrifugal pumps.
Pump Affinity Laws
Convert flow rate, head and power of centrifugal pumps for a changed speed or impeller diameter.
NPSH Calculator (Cavitation)
Calculate the available NPSH and cavitation margin of centrifugal pumps and check it against the required NPSH.