Revolutions per minute (rpm) measures rotational speed, expressing how many complete turns an object makes in one minute. It is the most common unit for engine speed, hard drive spin rates, and power tool specifications worldwide. One rpm equals exactly 1/60 hertz or approximately 0.01667 Hz.

Revolutions per second (rps) measures rotational speed as complete turns per second, equivalent to exactly one hertz. While less common than rpm in everyday specifications, rps is used in physics, engineering analysis, and some industrial equipment ratings. One rps equals exactly 60 rpm.