What fill rate measures
Fill rate answers a simple question: of everything customers asked for, how much did you actually ship right away from stock you already had? An item that was backordered, substituted, or partially shipped counts against fill rate even if the customer eventually got what they wanted, because the point of the metric is to isolate how well on-hand stock is covering real demand.
It is usually expressed as a percentage over a period — a week, a month, a quarter — and tracked per SKU, per category, or across the whole catalog.
Unit, line, and order fill rate
Unit fill rate is the simplest: units shipped ÷ units ordered. It treats every unit the same regardless of which order it belonged to.
Line fill rate looks at order lines instead of raw units: a line only counts as "filled" if the full quantity requested on that line shipped complete. This is stricter than unit fill rate, since partially shipping a line still counts as a miss.
Order fill rate is the strictest of the three: an order only counts as filled if every line on it shipped complete. A single out-of-stock item on an otherwise-complete 10-line order drops that entire order to a miss under this measure. Order fill rate is usually the number that best reflects what a customer actually experienced.
- Unit fill rate: forgiving, good for high-level trend tracking.
- Line fill rate: catches partial shipments per SKU line.
- Order fill rate: strictest, closest to actual customer experience.
How to improve fill rate
A persistently low fill rate on a specific SKU is almost always a reorder point or safety stock problem: either the reorder point is set below what lead-time demand actually requires, or the safety stock buffer is too thin for how variable that SKU's demand really is. Recalculating both against recent, not historical, demand and lead-time data is usually the fastest fix.
Fill rate can also expose supplier reliability issues that a raw stockout count would miss — a supplier who is chronically a few days late on lead time will quietly erode fill rate on every SKU they supply, even if outright stockouts are rare.
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