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.
Standards 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
ω = 2 π f , f = ω 2 π . {\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.
1 rad s = 1 2 π Hz = 60 2 π rpm 2 π rad s = 1 Hz = 60 rpm 2 π 60 rad s = 1 60 Hz = 1 rpm {\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}}}
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