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Size the buffer that protects you from demand and lead-time swings.
Safety stock is the price you pay for uncertainty. If demand were perfectly steady and suppliers perfectly punctual, the correct buffer would be zero. It is not zero because both vary — so the question is never "do we hold a buffer" but "how large a buffer buys the service level we promised, without burying cash in stock nobody asked for".
Safety stock
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Hold this many units as a buffer above expected demand.
Your numbers
How much daily demand typically varies.
The formula
Safety stock = Z × σ_daily × √Lead time
Z is the service-level factor; σ is the standard deviation of daily demand.
What it means
Safety Stock — full definition
Read in the glossaryPick the service level you actually commit to. 95% means you accept running short in roughly one replenishment cycle in twenty.
Calculate the standard deviation of daily demand over a recent, representative period — a spreadsheet STDEV over daily issue quantities is enough.
Use the same lead time you feed the reorder point, so the two numbers stay consistent.
Service level is not linear in cost. Moving from 95% to 99% roughly doubles the buffer for the same item — worth it for an A-class line, rarely worth it for a C-class one.
This formula assumes lead time is stable and only demand varies. If your supplier is the unpredictable half, the buffer it produces will be too small.
Worked example
Daily demand averages 40 units with a standard deviation of 12, and lead time is 7 days. At 95% service, safety stock is 1.65 × 12 × √7 ≈ 52 units — about 1.3 days of demand. Pushing the same item to 99% service raises the Z factor to 2.33 and the buffer to 74 units. That is 22 extra units of permanently parked cash to close the last four points of service.
Tie it to ABC class. A-class items that anchor customer relationships justify 97–99%; B-class typically sit at 95%; C-class items often do fine at 90% because a short wait costs almost nothing.
Variability accumulates across the lead-time window, but it partly cancels out day to day. The square root captures that — doubling lead time raises required buffer by about 41%, not 100%.
For genuinely made-to-order items with no promised availability, yes. For anything you promise off the shelf, a zero buffer means every ordinary demand spike becomes a stockout.
This calculator works on one set of numbers. AWRA OpsHub keeps safety stock calculator results current across every item, supplier and location — automatically.
Find the stock level that should trigger your next order.
Measure the share of demand you actually served — by unit and by order line.
Set the minimum and maximum stock levels that drive periodic-review ordering.