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 800003:2019 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}).^{[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:
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}.
