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Safety Stock and Reorder Point Calculator — result sheet
Estimate demand variability during lead time, safety stock, and a variability-aware reorder point from demand and lead-time inputs.
Inputs used
Results
Visual chart
Breakdown
Calculation steps
Returned data table
Formula and methodology
Formula: Lead-time demand deviation = √(average lead time × daily demand deviation² + average daily demand² × lead-time deviation²); safety stock = z × lead-time demand deviation; reorder point = average daily demand × average lead time + safety stock.
This inventory-planning model adds uncertainty from both demand and lead time. It is more informative than a rate-only reorder point when variability is measured, but it remains a statistical scenario rather than a stocking recommendation. The visitor supplies the service-level z-score and all units.
This result follows the calculator's declared inputs, precision, validation boundaries, and model limits.
Input contract
- Average daily demand — units/day; minimum 1.0E-6; maximum 1000000000
- Daily demand standard deviation — units/day; minimum 0; maximum 1000000000
- Average lead time — days; minimum 0; maximum 3650
- Lead-time standard deviation — days; minimum 0; maximum 3650
- Service-level z-score — Enter a z-score from your chosen service-level convention; the tool does not map a country or policy to a target.; minimum 0; maximum 10
Worked example
| Input | Value |
|---|---|
| Average daily demand | 100 |
| Daily demand standard deviation | 20 |
| Average lead time | 7 |
| Lead-time standard deviation | 1 |
| Service-level z-score | 1.65 |
Lead-time demand is 700 units; the variability-aware safety stock is about 99.9 units and the reorder point is about 799.9 units.
Assumptions and limits
- Demand observations are summarized by an average and standard deviation on a daily basis.
- Lead time is summarized by an average and standard deviation in days.
- The formula treats the two variability sources as independent in the combined variance approximation.
- The selected z-score represents a planning target supplied by the visitor.
- Demand and lead-time distributions are treated as sufficiently regular for a normal-style approximation.
- Seasonality, promotions, trends, lost sales, minimum orders, case packs, and supplier constraints are not modeled.
- The output is a continuous quantity; the business must round to usable units or packs.
- A higher z-score raises safety stock but does not guarantee a real-world fill rate.
- The result does not replace item-level review, service policy, or a current inventory system.
Calculator note
Source and methodology
Use the official WorldCalculate methodology policy for the source, formula, precision, and boundary standards behind this calculator.
Planning estimate, not financial, medical, legal, or professional advice. © WorldCalculate — reuse with attribution. Built and curated by Hassan ALRowaie.
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