Determine optimal buffer stock levels by comparing costs and coverage across different service levels.
Z = Z-score corresponding to the desired service level
σ(demand) = Standard deviation of daily demand
Lead Time = Supplier lead time in days
Enter your demand and lead time data above to see safety stock recommendations across service levels. Results update automatically as you type.
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Safety stock is the extra inventory held beyond expected demand to buffer against uncertainty in demand and supply lead times. It acts as insurance against stockouts when actual demand exceeds forecasts or deliveries are delayed.
Safety stock is calculated using the formula: Safety Stock = Z-score × Standard Deviation of Demand × √(Lead Time). The Z-score corresponds to your desired service level: higher service levels require more safety stock.
A service level represents the probability of not running out of stock during a replenishment cycle. Common choices are 90% (cost-conscious), 95% (balanced, recommended for most products), and 99% (critical items). Higher service levels require exponentially more safety stock.
The Z-score is a statistical value that corresponds to your desired service level. For a 90% service level, Z = 1.282; for 95%, Z = 1.645; and for 99%, Z = 2.326. It determines how many standard deviations of buffer you maintain.
Holding cost is the annual expense of storing one unit of inventory, typically 20-30% of the unit cost. Higher safety stock means higher holding costs, so it is important to balance the cost of carrying extra inventory against the cost of potential stockouts.