Safety Stock Calculator
SS = z × √(P·σd² + d̄²·σL²). Set your demand and lead-time statistics, choose a service level, and see the buffer the variability demands — and which uncertainty is driving it.
Try
SS = z × √( P·σd² + d̄²·σL² ) — King (APICS, 2011)
If you forecast, use the RMSE of forecast error instead of raw demand σ.
0 = continuous review. Otherwise, how often you check and reorder.
Optional: fill rate & naive baseline
Enables the fill-rate estimate (share of demand served from stock).
Both maxima enable the max-min heuristic row — for comparison only.
Safety stock—
Reorder point—
Safety factor z—σ of protection
Implied fill rate—of demand served
Where your risk lives (share of variance)
Demand variability —Lead-time variability —
Formula comparison
| Approach | When it applies | Safety stock |
|---|---|---|
| Max-min heuristic | Naive baseline — no service-level control, outlier-sensitive | — |
| Demand variability only | Lead time reliable (σL ≈ 0) | — |
| Lead-time variability only | Demand steady (σd ≈ 0) | — |
| Combined (King’s formula) | Both vary, independently — recommended | — |
| Linear sum | Demand and lead time move together (conservative bound) | — |
New to the concept? Read the essay on what safety stock really protects against — and the day every butter vendor ran dry.