Safety stock is the buffer between “in stock” and “sold out”: the units that absorb a late delivery or a sudden rush. This safety stock calculator leads with the formula most small sellers can actually feed — the simple maximum-and-average method, which needs only four numbers you already know — and shows the statistical Z method next to it for comparison, so you can see what the textbook approach would hold with your data.
La versión completa en español está aquí: Calcular Stock de Seguridad — Fórmula Simple. If you prefer the full statistical treatment in Spanish (standard deviations of demand and lead time), it lives in the companion tool linked in the related section below.
The simple formula
Safety stock = (maximum lead time × maximum daily sales) − (average lead time × average daily sales). The first bracket is the worst realistic stretch: your slowest delivery meeting your busiest selling day. The second bracket is a normal replenishment cycle. The difference is the buffer that has to cover the gap between “normal” and “bad week”. It needs no statistics — your longest observed lead time and your best sales day are in your order history — which is exactly why it is the method most small operations can maintain honestly.
Worked example
Longest lead time 10 days, busiest day 60 units, average lead time 6 days, average sales 40 units a day: (10 × 60) − (6 × 40) = 600 − 240 = 360 units of safety stock. The Z method on the same product — 95% service level (Z = 1.65), daily sales variation σ = 12 units, average lead 6 days — gives 1.65 × 12 × √6 = 48.5 units. The gap is real and expected: the simple method insures against the worst day you have actually seen, while the Z method insures against a statistical share of variation. Neither is “wrong”; they budget for different risks, and the simple method is deliberately conservative.
Choosing between them
Use the simple method when your data is thin, your supplier is erratic, or a stockout is expensive (the product drives traffic or anchors orders). Use the Z method when you have months of clean daily sales, a reliable supplier, and expensive shelf space that punishes over-buffering. What you should not do is average the two or pick the smaller because it looks cheaper: understated safety stock does not save money, it converts it into lost sales that never appear in any report. And whichever you choose, recompute when anything changes — a new supplier, a promotion rhythm, a seasonal peak — because a buffer calculated on last year’s lead times protects against last year’s problems.
Where the number goes next
Safety stock is not a target to hold for its own sake; it is an input. Add it to average daily sales × lead time and you have the reorder point — the stock level at which you place the next order — which the reorder point calculator linked below computes, accepting this page’s simple figure directly.
Where the four numbers come from
Average daily sales: divide the last 30–90 days of unit sales by the number of days — use a window that matches how you actually sell, and exclude freak one-off bulk orders or the average inflates. Maximum daily sales: the best single day in that same window, or a realistic busy-day figure if one viral day distorts it. Average lead time: the mean of your last several deliveries from this supplier, in calendar days as the customer experiences them. Maximum lead time: the slowest of those deliveries — not the slowest imaginable, the slowest you have evidence for. Review the set monthly for your top sellers; a buffer built on a supplier’s old warehouse location protects against a supply chain you no longer have.
Related tools
Versión en español: Calcular Stock de Seguridad: Fórmula Simple de Máximos y Medias, sin Estadística · Reorder Point Calculator · Inventory Turnover Calculator · Safety stock — the statistical method (in Spanish).
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