Surplus vs Shortage
Too much stock versus too little, and the cost of each.
Definition
A surplus is having more inventory than demand requires; a shortage is having less than demand requires. Both are costly: a surplus ties up cash and storage and risks markdowns, while a shortage means lost sales and, on Amazon, lost ranking.
The cost of a surplus
Cash frozen in units that are not selling, monthly storage plus eventual aged-inventory surcharges, a heavier excess-inventory ratio dragging your IPI, and the markdowns or removals it takes to clear whatever will not move on its own.
The cost of a shortage
On Amazon a shortage does not become a backorder, it becomes a lost sale, lost ranking momentum, possible exposure to the low-inventory-level fee, and a customer who just bought the same thing from your competitor. The damage outlasts the stockout, because rank is hard to win back.
How to stay between surplus and shortage
A per-SKU forecast, safety stock sized to a target service level, and a reorder point that fires on time. The practical tell is variability: a SKU that sells a steady 20 a day tolerates a simple weeks-of-cover rule, but a spiky or seasonal SKU whose daily sales swing widely needs its own safety stock and reorder math, and that is exactly where the blanket rule drops you into a surplus or a shortage.
Related terms
Stay in the band between too much and too little
Inventory Hero sizes each SKU's reorder point and safety stock to its real demand, so you avoid both the storage bill of a surplus and the lost sales of a shortage.