Free Tool
Safety Stock Calculator
Determine how much buffer inventory you need to protect against stockouts. This calculator uses the King formula which accounts for both demand and lead time variability.
Enter demand, std deviation, and lead time to see results
Frequently Asked Questions About Safety Stock
What is safety stock?
Safety stock is extra inventory held as a buffer against demand variability and supply uncertainty. It protects your business from stockouts when actual demand exceeds forecasts or when supplier lead times are longer than expected. For distributors managing thousands of SKUs across multiple vendors, safety stock is the difference between a smooth operation and lost customers. The amount of safety stock you need depends on your demand variability, lead time variability, and target service level.
How do you calculate safety stock?
The most accurate method is the King formula: Safety Stock = Z × √(LT × σd² + d² × σLT²), where Z is the service level Z-score (1.65 for 95%, 2.33 for 99%), LT is average lead time, σd is demand standard deviation, d is average demand, and σLT is lead time standard deviation. A simpler alternative is: (Max Daily Sales × Max Lead Time) − (Avg Daily Sales × Avg Lead Time). Use the calculator above to run either formula with your actual numbers.
What is the safety stock formula?
The standard safety stock formula is SS = Z × σdLT, where Z is the Z-score for your desired service level and σdLT is the standard deviation of demand during lead time. The more comprehensive King formula extends this to account for lead time variability: SS = Z × √(LT × σd² + d² × σLT²). This calculator uses the King formula because lead time variability is a major factor for distributors working with multiple vendors. Read our full safety stock calculation guide for detailed examples.
How much safety stock should I carry?
The right amount depends on the SKU's importance. Use ABC classification to segment your catalog: A-tier items (top 20% by revenue) should target 98-99% service level with generous safety stock. B-tier items can run at 95%. C-tier slow movers can target 90% or be ordered on demand. This tiered approach can reduce total safety stock investment by 20-30% while actually improving fill rates on your most important products.
What happens if safety stock is set too high or too low?
If safety stock is too low, you risk stockouts, lost sales, rush shipping costs, and damaged customer relationships. In distribution, a single stockout on a critical part can send an entire order to a competitor permanently. If safety stock is too high, you tie up working capital in excess inventory, increase carrying costs (typically 20-30% of inventory value per year), and risk obsolescence. The goal is to use statistical methods — not gut feel — to balance stockout risk against holding cost for each SKU individually.
Calculate This for Every SKU Automatically
Tru-Stock AI calculates safety stock for thousands of products simultaneously using your actual sales data and vendor lead times.