Safety Stock Calculator
Calculate safety stock using demand variability, lead time variability, and your target service level to protect against stockouts without over-investing in inventory.
Formula & Reference
| Variable | Symbol | Formula | Units |
|---|---|---|---|
| Safety Stock Calculator | — | Safety stock = Z × √(LT × σd² + d² × σLT²) | units |
Step-by-Step Examples
Demand 40/day (SD 12), lead time 14 days (SD 3), 95% service level.
- Demand variance term = 14 × 12² = 2,016
- Lead time term = 40² × 3² = 14,400
- Total variance = 16,416, √ = 128.1
- Safety stock = 1.65 × 128.1 = 211 units
Demand 40/day (SD 12), lead time 14 days (SD 0), 95% service.
- Lead time term = 0
- Variance = 14 × 144 = 2,016, √ = 44.9
- Safety stock = 1.65 × 44.9 = 74.1, rounded up to 75 units
- Eliminating lead time variability cut safety stock by nearly two thirds
Same as example 1 but targeting 99.9%.
- Same variance, √ = 128.1
- Safety stock = 3.09 × 128.1 = 396 units
- Going from 95% to 99.9% nearly doubles the inventory required
Real-World Applications
Common Mistakes to Avoid
A fixed days-of-cover rule ignores variability entirely. Two products with identical demand but different volatility need very different buffers.
As the examples show, lead time variability often contributes more than demand variability. Focusing only on forecast accuracy misses the larger lever.
The Z-multiplier approaches infinity as service level approaches 100%. Perfect availability is mathematically impossible to guarantee with finite stock.