Safety Stock Calculator
Work out how much buffer stock each SKU needs — two methods, instant results, no sign-up. Start with the average–max method from your own worst-case sales and lead times, or set an explicit availability target with the Z-score service-level method.
Method 1: Average–Max Safety Stock
SS = (max daily sales × max lead time) − (average daily sales × average lead time). Uses your own worst observed case — simple and data-driven.
Use stockout-adjusted sales history and delivered (not quoted) lead times — otherwise the buffer inherits the errors it exists to absorb.
Method 2: Z-Score (Service Level) Safety Stock
SS = Z × σd × √L. Choose the availability you want — 90% → Z 1.28, 95% → 1.65, 98% → 2.05, 99% → 2.33 — and the buffer scales with your real demand volatility.
σd is the standard deviation of daily demand over a representative period. Higher service levels cost exponentially more buffer — reserve 98%+ for A-class SKUs.
The formula, and a worked example
Safety stock is the buffer held on top of expected lead-time demand to absorb two kinds of surprise: demand running hotter than forecast, and suppliers delivering later than promised. The average–max method sizes it from your own worst observed case; the Z-score method sizes it statistically from demand volatility and a chosen service level.
Worked example (average–max): a SKU sells 6 units/day on average with an 18-day average lead time, but sales have peaked at 10/day and lead time has stretched to 25 days. Safety stock = (10 × 25) − (6 × 18) = 250 − 108 = 142 units. The reorder point becomes 108 + 142 = 250 units — order when stock falls to that level and the buffer should still be on the shelf when the delivery lands.
For the full theory — all four methods, choosing Z-scores, and keeping the number honest at scale — read the safety stock formula guide. Safety stock then plugs into the reorder point calculator to set your order trigger.
Calculating this once is arithmetic; keeping it current for thousands of SKUs as velocity, seasonality and supplier behaviour drift is a software job. Inventory replenishment software recalculates safety stock and reorder points continuously from live Shopify and Linnworks data — and raises the purchase orders they trigger.
Stop Calculating This By Hand
Replenagise sizes safety stock per SKU from live demand variability and lead times — recalculated continuously, feeding straight into reorder points and purchase orders.