Safety Stock Calculator
Calculate safety stock from your demand variability, lead time, and target service level — including the combined formula for variable lead times.
How much daily sales swing. Estimate: (best day − worst day) ÷ 4.
Leave 0 if your supplier is reliably on time.
The z-score converts your service level into "how many standard deviations of demand to cover": 95% → 1.65, 99% → 2.33. Each extra "nine" costs disproportionately more stock — that's the real trade-off. Add this to lead-time demand in the reorder point calculator.
Frequently asked questions
What is the safety stock formula?
The standard formula is z × σ_demand × √(lead time): the z-score for your target service level, times the standard deviation of daily demand, times the square root of lead time in days. If lead time itself varies, use the combined form z × √(L·σd² + d²·σL²) — this calculator switches automatically.
What z-score should I use?
It comes from your target in-stock probability: 90% → 1.28, 95% → 1.65, 99% → 2.33, 99.9% → 3.09. Each additional "nine" costs disproportionately more stock, which is why very few products justify 99.9% service levels.
How do I estimate my demand standard deviation?
Export daily sales for the SKU over 60–90 days and compute the standard deviation in a spreadsheet (STDEV). Quick approximation if you only know the range: (best day − worst day) ÷ 4.
Is more safety stock always safer?
It trades stockout risk for carrying cost and obsolescence risk. The right level depends on the margin you lose in a stockout versus the holding cost — high-margin, hard-to-substitute products justify more buffer than commodity items.