Safety Stock Calculator
Calculate optimal buffer inventory to prevent stockouts while minimizing carrying costs. Enter your data below.
Enter Your Data
Units sold per day on average
How much daily demand varies (use STDEV in Excel)
% of demand you want to fulfill from stock
For calculating safety stock value
Results
How to Use This Calculator
This safety stock calculator uses the standard formula that accounts for both demand variability and lead time variability:
The Formula
Safety Stock = Z × √(LT × σD² + D² × σLT²)
Where:
- Z = Service level factor (Z-score)
- LT = Average lead time in days
- σD = Standard deviation of daily demand
- D = Average daily demand
- σLT = Standard deviation of lead time
Service Level Z-Values
| Service Level | Z-Value | Meaning |
|---|---|---|
| 90% | 1.28 | 10% chance of stockout per cycle |
| 95% | 1.65 | 5% chance of stockout per cycle |
| 97% | 1.88 | 3% chance of stockout per cycle |
| 98% | 2.05 | 2% chance of stockout per cycle |
| 99% | 2.33 | 1% chance of stockout per cycle |
How to Find Your Standard Deviation
In Excel, use the =STDEV() function on your historical daily demand data. For example, if your daily sales for the past 30 days are in cells A1:A30, use =STDEV(A1:A30).
Need More Than a Calculator?
This calculator gives you a quick answer, but real inventory optimization requires:
- Automated calculations across all your SKUs
- Real-time data from your ERP/WMS
- AI-powered demand forecasting
- Alerts when safety stock levels need adjustment
Get a free assessment to see how Flair Group can automate this for your entire inventory.