Häufig gestellte Fragen
What is safety stock?
Safety stock (in German: Sicherheitsbestand) is an iron reserve of inventory that a company holds over and above its planned requirements in order to cushion fluctuations in demand and replenishment. It serves as a buffer in case consumption turns out higher than forecast or a delivery is delayed. In ERP and materials management systems it is a central master data parameter of every material and feeds directly into order quantity and scheduling decisions. It reduces the probability of shortages but does not eliminate them entirely.
How do you calculate safety stock?
Several approaches exist for determining it, from a flat allowance covering a certain number of days of consumption to statistical methods. A widespread statistical formula for the case of fluctuating demand with a constant replenishment lead time is safety stock = safety factor × standard deviation of demand × square root of the replenishment lead time. The safety factor (z-value) is derived from the target service level via the normal distribution, for example around 1.65 for 95 percent or 2.33 for 99 percent. The required buffer therefore grows with demand variability, the length of the replenishment lead time and the desired service level.
What is the difference between safety stock and reorder point?
Safety stock describes the pure reserve that is meant to remain untouched as a buffer. The reorder point, by contrast, is the inventory threshold below which the system triggers a replenishment order. The reorder point contains the safety stock as a component plus the expected consumption during the replenishment lead time, commonly expressed as the formula reorder point = safety stock + average daily consumption × replenishment lead time. Ideally, the replenishment delivery therefore arrives exactly when the stock has been drawn down to the safety stock level.
How high should the safety stock be?
There is no blanket answer to the right level; it results from the interplay of service level, demand variability and delivery reliability. The higher the target service level, the disproportionately larger the buffer becomes, because the last percentage points of availability come at a particularly high price. An upstream ABC analysis helps to concentrate scarce reserves on revenue-critical A items and to calculate more sparingly for low-value items. Moreover, the value should not be fixed statically but adjusted regularly to current consumption and lead time data.
Does a high safety stock cause costs?
Yes, a high safety stock increases availability but ties up capital and causes warehousing, insurance and, where applicable, scrapping costs. In practice, these ongoing inventory costs are frequently set as a percentage of the inventory value per year, often in the order of about 20 to 30 percent, and add up considerably for a permanently held reserve. Stock that is too low lowers these costs but risks production stoppages or lost sales due to shortages. The optimal level is therefore always a deliberately calculated trade-off between service level and capital commitment.
Can safety stock be avoided entirely with just-in-time?
In just-in-time-oriented concepts, safety stock is deliberately minimised to reduce capital commitment and warehousing costs, but this presupposes very high delivery and process reliability. If that reliability fails, for instance due to supply bottlenecks or logistics disruptions, the buffer is missing and production stoppages quickly follow. The supply chain disruptions of the pandemic years showed that purely lean inventories can be fragile, which is why many companies have since relied more heavily on a just-in-case logic with targeted reserves. In practice, a hybrid approach is often sensible, combining a reduced but not completely abolished safety stock with JIT principles.