A duty cycle or power cycle is the fraction of one period in which a signal or system is active. Duty cycle is commonly expressed as a percentage or a ratio. A period is the time it takes for a signal to complete an on-and-off cycle. As a formula, a duty cycle (%) may be expressed as:

${\displaystyle D={\frac {PW}{T}}\times 100\%}$

Equally, a duty cycle (ratio) may be expressed as:

${\displaystyle D={\frac {PW}{T}}}$

where ${\displaystyle D}$ is the duty cycle, ${\displaystyle PW}$ is the pulse width (pulse active time), and ${\displaystyle T}$ is the total period of the signal. Thus, a 60% duty cycle means the signal is on 60% of the time but off 40% of the time. The "on time" for a 60% duty cycle could be a fraction of a second, a day, or even a week, depending on the length of the period.

Duty cycles can be used to describe the percent time of an active signal in an electrical device such as the power switch in a switching power supply or the firing of action potentials by a living system such as a neuron.

The duty factor for periodic signal expresses the same notion; it is usually expressed as a fraction rather than as a percentage.

The duty cycle can also be notated as ${\displaystyle \alpha }$.