Safety Stock
Definition
Extra inventory held beyond expected demand to protect against variability in demand or supply. It acts as a buffer that keeps orders flowing when forecasts miss or deliveries run late.
In Practice
Safety stock is sized using demand variability, supply lead time variability, and a target service level. The classic formula multiplies a service-level Z-score by the standard deviation of demand over lead time, so items with erratic demand or unreliable suppliers carry proportionally more buffer.
For a planner, safety stock is the lever that trades working capital against stockout risk. Set it too low and every forecast miss becomes a backorder; set it too high and cash sits on shelves while carrying costs climb. Reviewing safety stock quarterly as demand patterns shift is a core planning discipline.
Example: a distributor sells an average of 100 units per day of a fast-moving SKU with a 10-day supplier lead time. Historical variability suggests demand over lead time can swing by 150 units, so the planner holds roughly 250 units of safety stock to hit a 98 percent service level.
Related Calculators
Related Terms
The inventory level that triggers a replenishment order. It equals expected demand during supplier lead time plus safety stock.
Service LevelThe target probability of not stocking out during a replenishment cycle, or more broadly the standard of product availability promised to customers. It is the key input for sizing safety stock.
Buffer StockInventory held specifically to absorb shocks, whether from demand surges, supply delays, or process disruptions. The term is often used interchangeably with safety stock, though it can refer to any protective stock positioned in a process or network.
Demand VariabilityThe degree to which actual demand fluctuates around its average over time, commonly measured by standard deviation or the coefficient of variation. It is the primary driver of how much safety stock an item needs.