Rotations per minute is a unit of frequency used to measure the number of rotations about a fixed axis in one minute. It is frequently used to measure the angular velocity of rotating mechanical components such as wheels.
Common examples of the unit RPM include automobile engine speed idling around 750 RPM or a computer hard drive operating at 5400 RPM. RPM is not recognized as a unit in the International System of Units but is used frequently around the world. The SI unit for frequency is Hz, although radians per second is also used. Frequency can be documented as the number of occurrences of a period of time or the rate of change of phase of a sinusoidal waveform.