${\displaystyle D=\sum {\frac {n_{i}}{N_{i}}}}$
${\displaystyle D=}$ a constant, often ranging between 0.33 and 3
${\displaystyle n_{i}=}$ the number of occurrences of a specific load cycle progression, ${\displaystyle i}$
${\displaystyle N_{i}=}$ the fatigue life of a specific load cycle progression, ${\displaystyle i}$