Inventory Planning, Forecasting and the Bullwhip

Inventory is the supply chain's shock absorber and its most expensive mistake. Every company holds it for the same reasons (factories run in batches, ships take weeks, customers want things now) and every company pays for it the same way: in cash tied up, in warehouse space, in obsolescence and markdowns, and in interest, typically adding up to ~20–30% of the inventory's value a year on common rules of thumb (verify for any business). The amount the US economy holds relative to its sales moves with the cycle: the Census Bureau's total business inventories-to-sales ratio fell to unusually low levels in 2021 as demand outran supply (around ~1.25 in mid-2021; verify), rebuilt through 2022–23, and stood at ~1.33 in August 2026, down from ~1.40 a year earlier (Census release of 15 Oct 2026, reported), as companies that had stockpiled ahead of the 2025 tariffs worked the stock down. The pandemic years showed the bullwhip effect at national scale: small changes in what consumers bought turned into much larger swings in what retailers ordered from wholesalers, what wholesalers ordered from factories and what factories ordered from their suppliers. Shortages in 2021 became gluts in 2022: Target warned in June 2022 that it would cut prices and cancel orders to clear excess inventory, Walmart and Nike reported inventories up by a third or more year on year, and Peloton went from waiting lists to write-downs within a year. This section explains how inventory is measured and planned, how forecasts are made and judged, why the bullwhip happens and how to damp it, the 2020–23 shortage-to-glut cycle, just-in-time against just-in-case, and inventory as working capital. Two companion files give a four-tier bullwhip simulation and an inventory-health review checklist.