Safety Stock Calculator
Calculate optimal safety stock across service levels, demand variability, and lead time uncertainty. Balance stockout risk against the working capital cost of holding buffer inventory.
Service level comparison
| Service level | Safety stock units | Holding cost per year | Stockout risk |
|---|---|---|---|
| 80% | 36 | $108 | 20% |
| 85% | 44 | $132 | 15% |
| 90% | 55 | $165 | 10% |
| 95% | 70 | $210 | 5% |
| 97% | 80 | $240 | 3% |
| 99% | 99 | $297 | 1% |
Two ways to size safety stock
- Statistical method
z-score x square root of (avg lead time x demand std squared + avg demand squared x lead time std squared), rounded up. It combines demand swings and lead time swings at the service level you choose.
- Simple method
(Max daily demand minus average daily demand) x max lead time. It assumes your worst demand day lasts through your longest lead time, so it usually holds far more stock than the statistical method.
Each step up in service level costs more buffer than the last: in the default example, going from 95% to 99% adds 29 units (70 to 99), while 90% to 95% adds 15.
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The tools are free. QVINTA does the work.
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