Safety Stock Calculator
Safety stock is the buffer inventory held above expected demand to protect against stockouts when demand is unexpectedly high or a supplier delivery is late. The most statistically rigorous approach uses standard deviations of both demand and lead time, combined with a Z-score corresponding to the desired service level. This calculator implements the combined formula that accounts for variability in both demand and lead time, which is more accurate than the simpler demand-only or lead-time-only versions. Enter your average and standard deviation of daily demand, average and standard deviation of lead time, and your target service level to compute the required safety stock.
Safety stock formula
Safety Stock = Z * sqrt(Avg Lead Time * Demand Std Dev^2 + Avg Demand^2 * Lead Time Std Dev^2)
Annual Holding Cost = Safety Stock * Unit Value * (Holding Rate / 100)
Z-scores for common service levels
- 90% service level: Z = 1.28
- 95% service level: Z = 1.645
- 97.5% service level: Z = 1.96
- 99% service level: Z = 2.326
- 99.5% service level: Z = 2.576
Safety stock: frequently asked questions
What is safety stock?
Safety stock is the extra inventory held to protect against stockouts caused by unexpected demand increases or supplier delays. It is a buffer above the cycle stock (the inventory needed to meet expected demand during the lead time).
How is safety stock calculated?
The standard formula is: Safety Stock = Z * sqrt(Avg Lead Time * Demand Std Dev^2 + Avg Demand^2 * Lead Time Std Dev^2). For simplicity with fixed lead times: Safety Stock = Z * Lead Time Std Dev * Avg Demand or Z * Demand Std Dev * sqrt(Avg Lead Time).
What Z-score should I use for safety stock?
Z-score depends on the desired service level. For 90% service level, Z = 1.28. For 95%, Z = 1.65. For 99%, Z = 2.33. A higher service level requires more safety stock and higher holding costs.
What is a service level in inventory management?
Service level is the probability of not stocking out during any given replenishment cycle. A 95% service level means there is a 95% probability that you will have enough stock to fill all orders during the lead time period.
What is the cost of holding safety stock?
Safety stock holding cost = Safety Stock Units * Unit Value * Annual Holding Cost Rate. If safety stock is 200 units at $10 each with a 25% annual holding rate, the annual carrying cost is $500. This must be weighed against the cost of stockouts.
Sources
- U.S. Small Business Administration: Manage Your Business Finances.
- U.S. Census Bureau: Manufacturing Inventories Survey.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.