NOTE 1 The rotational frequency is also the derivative of the absolute value of angle of rotation φ with respect to time t, divided by 2π, thus ${\displaystyle n=\frac{1}{2\pi }\frac{\text{d}\left|\phi \right|}{\text{d}t}}$.

NOTE 2 The coherent SI unit of rotational frequency is second to the power minus one, s^–1. The units revolution per second, symbol r/s, and revolution per minute, symbol r/min, are widely used in specifications for rotating machines.