The Safety Stock Calculator estimates buffer inventory needed to protect against demand and lead-time variability. It supports a service-factor method using standard deviations and also reports a reorder point based on average demand during lead time plus the calculated safety stock.
Inventory planners can use it to test service levels, supplier reliability, and demand volatility. The model assumes the demand and lead-time units are compatible, and the output should be balanced against carrying cost, shelf life, and storage constraints.
1. Enter average daily demand
Use a representative demand period and the same unit across all inputs.
2. Measure demand variability
Enter the standard deviation of daily demand.
3. Enter lead-time statistics
Use average supplier lead time and its standard deviation in days.
4. Choose a service factor
Enter the Z-value associated with the service level being modeled.
5. Review stock and reorder point
Round operational quantities according to pack sizes and purchasing rules.
This combined-variability model assumes demand and lead-time variation are independent and approximately stable.
Safety stock is the inventory buffer above expected lead-time demand; the reorder point indicates when replenishment should be triggered.
Promotions, minimum order quantities, intermittent demand, spoilage, and correlated supplier delays may require a different model.
Given: Average demand 120 units/day, demand SD 25, average lead time 14 days, lead-time SD 2 days, and Z = 1.65.
Calculation: Safety stock = 1.65 × √[(14 × 25²) + (120² × 2²)] = 425.86, rounded up to 426 units. Average lead-time demand = 120 × 14 = 1,680 units. Reorder point = 2,105.86, rounded to 2,106 units.
Result: Hold about 426 safety units and reorder near 2,106 units.
It is a Z-value representing the desired cycle service level under a normal-distribution assumption.
Yes, but convert lead time and its variability to weeks so all time units match.
Either source can create a stockout, so the combined formula reflects uncertainty in both.
Operationally, round up to whole units and then adjust for case packs or minimum order quantities.
No. Safety stock is the buffer; the reorder point includes expected demand during lead time plus that buffer.