Safety Stock Calculator

What Is Safety Stock?

Safety stock is the buffer inventory you carry beyond the expected demand during replenishment lead time. Its job is to absorb the two things that can go wrong: demand spiking higher than average (σD), or lead time running longer than average (σLT). The target service level is the probability you want to avoid a stockout during a replenishment cycle — 95% means you accept stocking out in roughly 1 of every 20 cycles.

The Formula Most People Get Wrong

The correct formula accounts for BOTH sources of variability: SS = Z × √(LT·σD² + D²·σLT²). Many textbooks show only the demand-variability term Z·σD·√LT, which is fine when supplier lead times are metronomic but dangerous when they are not. For imported goods, seasonal ocean rates, or suppliers with frequent delays, the lead-time term often dominates — and ignoring it produces buffers that are too small. This calculator always uses the full formula; set σLT to 0 to see the demand-only case.

Reorder Point

The reorder point is the stock level that triggers a new purchase order: expected demand during lead time plus safety stock. When on-hand inventory drops to this level, place an order of your Economic Order Quantity (see the EOQ Calculator). The goal is to receive the replenishment just before the safety stock is consumed.

Picking a Service Level

Higher service levels rapidly increase safety stock — the curve is exponential in the tails. Going from 95% to 99% might double your buffer; going to 99.9% can triple it. For A-tier items and critical components, 90–95% is usually too lax; for slow-moving C-items, 99% is often wasteful. A good first pass is 95% for most of the assortment and 98–99% for items whose stockout cost (lost sale, line-down, customer churn) is unusually high. I help retailers and distributors tier their assortment and tune service levels per segment. Learn about integration, custom reporting, or get in touch.