Quick Answer: In computing, clock rate or clock speed is the frequency at which the clock generator of a processor can generate pulses used to synchronize the operations of its components. It is used as an indicator of the processor speed. Clock rate is measured in the SI unit of frequency hertz. The clock rate of the first generation of computers was measured in hertz or kilohertz, the first personal computers from the 1970s through the 1980s had clock rates measured in megahertz. In the 21st century the speed of modern CPUs is commonly advertised in gigahertz. This metric is most useful when comparing processors within the same family, holding constant other features that may affect performance.