Order Fill Rate

Full Definition

Order fill rate measures what fraction of the total units customers order can be shipped immediately from available inventory. If customers order 10,000 units across a period and 9,400 units are shipped from stock, the fill rate is 94%. The remaining 6% represents backorders—items that must be sourced or produced after the original order date and shipped separately, creating additional cost and customer inconvenience. Fill rate is the fundamental metric connecting inventory management performance to customer service outcomes, making it a critical KPI for any company that holds inventory to serve customer demand. Fill rate calculation must be defined consistently—the metric can be measured at the unit level (total units shipped divided by total units ordered), line level (order lines shipped complete divided by total order lines), or order level (complete orders shipped divided by total orders). These measures differ significantly: a 95% unit fill rate may correspond to a 70% order fill rate if stockouts tend to occur on items that appear on many orders. The order-level fill rate is most relevant for customer experience; the unit fill rate is most relevant for revenue impact analysis. Fill rate targets reflect a deliberate trade-off between service level and inventory investment. Every percentage point of fill rate improvement requires holding more safety stock, which increases working capital and storage cost. Econometric models estimate the required inventory investment to achieve each service level target, enabling management to make an informed decision about the appropriate service level rather than defaulting to "as high as possible." For many businesses, the optimal strategy involves setting different fill rate targets by product category—near-100% for high-velocity, high-margin items where stockouts directly impact revenue, and lower targets for slow-moving specialty items where carrying cost outweighs the service benefit.

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