A stockout is what happens when available inventory reaches zero while customers still want to buy. The order cannot be fulfilled, the product page goes dark, and the customer goes elsewhere often for good. That single event costs more than the immediate lost sale. According to a 2023 analysis by IHL Group, inventory distortion of which stockouts are the dominant component cost global retailers $1.77 trillion, equivalent to 7.2% of all retail sales. For ecommerce brands, the number is compounded by a second hit: marketplace algorithms penalise inconsistent availability, suppressing your ranking for weeks or months after you restock, long after the shelf is full again. This guide covers what a stockout is, what causes it, how to calculate the