If a centrifugal pump has an impeller speed of $N$ (in rpm), discharge $Q$ (in $m^3/s$) and the total head $H$ (in $m$), the expression for the specific speed $N_s$ of the pump is given by

- $N_s = \dfrac{NQ^{0.5}}{H^{0.5}} \\$
- $N_s = \dfrac{NQ^{0.5}}{H}\\$
- $N_s = \dfrac{NQ^{0.5}}{H^{0.75}} \\$
- $N_s = \dfrac{NQ}{H^{0.5}}$