Duty cycle calculation is done by taking the time a waveform is high and dividing that by the period of the waveform. In other words, duty cycle is a ratio of time high to period of a waveform.

Three different waveforms are presented below.
#1 shows a waveform with equal time high and low. This waveform is at 50% duty cycle.
#2 shows a waveform with 3x time high than low. This waveform is at 75% duty cycle.
#3 shows a waveform with 3x time low than high. This waveform is at 25% duty cycle.