[h=Specific speed may be determined independent of the pump size]2[/h]
Specific speed is a number characterizing the type of impeller in a unique and coherent manner. Specific speed are determined independent of pump size and can be useful comparing different pump designs. The specific speed identifies the geometrically similarity of pumps.
Specific speed can be expressed as
Ns = ω q1/2 / h3/4 (1)
where
Ns = specific speed
ω = pump shaft rotational speed (rpm)
q = flow rate (m3/h, l/s, l/min, m3/min, US gpm, British gpm) at Best Efficiency Point (BEP)

h = head rise (m, ft)