Derivation and Analysis of Fill Rate and Utilization Rate for Spare Parts

Inventory metrics are often expected to meet high standards. However, it is challenging to achieve high values for both fill rate (FR) and utilization rate (UR) simultaneously. In order to explain this phenomenon, this paper conducts some work. We derive the expression and linear relationship for these metrics based on stationary process, considering the specified initial stock level (ISL) and turnaround time. By adjusting the ISL, FR and UR exhibit opposite trends, which can explain the reason that both of them cannot achieve the requirements simultaneously. A numerical example is provided to illustrate the derived linear relationship and patterns of change. This paper aims to assist designers in setting more reasonable requirements for inventory metrics.