Safety Stock

Safety stock is the extra inventory held to protect against random demand swings while replenishment is on the way. The term Z scales the buffer to the desired service level, while σ√L gives the typical size of demand uncertainty accumulated over a lead time of L periods; because independent demand variation adds like a square root, longer lead times increase uncertainty more slowly than linearly. This formula is most appropriate when demand per period is roughly stable in mean, random in variation, and lead time is known or nearly fixed.

Use this when you need a statistically grounded inventory buffer for items with uncertain demand, especially in ordering systems where stockouts are costly. It is common in retail, distribution, spare-parts planning, and production supply for deciding how much extra inventory to carry above expected demand.

• Using the average demand instead of the standard deviation for σ
• Forgetting to take the square root of lead time and using L directly
• Applying the formula when lead time or demand is highly variable without adjusting the model