KEVOS® WORKOS · Inventory
Safety Stock
How much buffer stock should you hold?
Sizes safety stock by the statistical service-level method when demand and lead-time variation are known, and by the max-minus-average rule otherwise.
Open the calculatorRuns in your browser. No account needed.
How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Maximum daily demand | units/day | Highest normal daily demand in recent history |
| Maximum lead time | days | Longest normal lead time experienced |
| Average daily demand | units/day | Average daily demand |
| Average lead time | days | Average lead time in days |
| Std deviation of daily demand (0 = not known) | units/day | Standard deviation of daily demand from 6–12 months of history |
| Std deviation of lead time (0 = not known) | days | Standard deviation of lead time from receipt records |
| Target cycle service level | choice | Target probability of not stocking out in a replenishment cycle |
Method and formulas
- Statistical: SS = z × √(L × σd² + d² × σL²), with z from the service level (95% → 1.645)
- Rule of thumb: SS = max daily × max lead − average daily × average lead
Limitations
- The statistical method assumes independent, roughly normal demand
- Lumpy, seasonal or intermittent demand needs other methods
Typical uses
- Setting buffers for purchased and made items
- Reviewing service levels and stock-outs
Who it is for
- Inventory planners
- Supply chain analysts
What you get
- Safety stock in units and days
- Comparison of the two methods
Interpreting the result
- Higher service levels raise safety stock steeply above about 97.5%
- Without variation data, treat the result as a rule of thumb
References
- Silver E. A., Pyke D. F. and Thomas D. J., Inventory and Production Management in Supply Chains (CRC Press)
WORKOS tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.
