Safety Stock Calculator
Calculate statistical safety stock from demand variability, lead-time variability and service-factor assumptions using a combined-variance formula.
Calculate statistical safety stock
Estimate buffer stock when both daily demand and replenishment lead time can vary.
Safety stock formula with demand and lead-time variability
This calculator uses a common independent-variability model in which lead-time-demand standard deviation is √(average lead time × demand SD² + average demand² × lead-time SD²). Safety stock is that standard deviation multiplied by a selected z service factor. Oracle documents this combined-variability approach in inventory planning examples.
Illustrative reorder point
The displayed reorder point is average lead-time demand + safety stock. It assumes the demand/lead-time units are aligned and does not include order-review periods, minimum order quantities, pipeline-policy adjustments or known scheduled demand.
No single safety-stock formula fits every inventory system
IBM and other planning references note that safety-stock methods vary with the situation. Intermittent demand, non-normal demand, correlated demand/lead time, service-level definitions, perishability and multi-echelon networks can require different models or simulation.
Service factor is not a guaranteed fill rate
The z values correspond approximately to one-sided normal probabilities for a cycle-service interpretation. Fill rate and item-availability targets are different service concepts. Validate the selected service policy and estimate variability from clean, representative data rather than choosing a z value only because it looks conservative.