Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or r⋅min⁻¹) is a unit of rotational speed (or rotational frequency) for rotating machines. One revolution per minute is equivalent to ⁠1/60⁠ hertz.

ISO 80000-3:2019 defines a physical quantity called rotation (or number of revolutions), dimensionless, whose instantaneous rate of change is called rotational frequency (or rate of rotation), with units of reciprocal seconds (s⁻¹).

A related but distinct quantity for describing rotation is angular frequency (or angular speed, the magnitude of angular velocity), for which the SI unit is the radian per second (rad/s).

Although they have the same dimensions (reciprocal time) and base unit (s⁻¹), the hertz (Hz) and radians per second (rad/s) are special names used to express two different but proportional ISQ quantities: frequency and angular frequency, respectively. The conversions between a frequency f and an angular frequency ω are $${\displaystyle \omega =2\pi f,\quad f={\frac {\omega }{2\pi }}.}$$

Thus a disc rotating at 60 rpm is said to have an angular speed of 2π rad/s and a rotation frequency of 1 Hz.

The International System of Units (SI) does not recognize rpm as a unit. It defines units of angular frequency and angular velocity as rad s⁻¹, and units of frequency as Hz, equal to s⁻¹.

$${\displaystyle {\begin{array}{rcrcr}1~{\dfrac {\text{rad}}{\text{s}}}&=&{\dfrac {1}{2\pi }}~{\text{Hz}}&=&{\dfrac {60}{2\pi }}~{\text{rpm}}\\2\pi ~{\dfrac {\text{rad}}{\text{s}}}&=&1~{\text{Hz}}&=&60~{\text{rpm}}\\{\dfrac {2\pi }{60}}~{\dfrac {\text{rad}}{\text{s}}}&=&{\dfrac {1}{60}}~{\text{Hz}}&=&1~{\text{rpm}}\end{array}}}$$