Business
Safety Stock Calculator
Enter average and max daily demand, lead days and a Z service factor to see safety stock units.
How much safety stock do I need?
Safety stock is the extra inventory you keep so demand spikes or late deliveries do not empty the shelf before the next order arrives. A common planning form is safety stock ≈ Z × σd × √L, where σd is day-to-day demand variability, L is lead time in days and Z rises with the service level you want. This calculator approximates σd as the gap between maximum and average daily demand — blunt but usable when you lack a full forecast error history. Higher Z or longer lead times demand more buffer. Feed the result into reorder thinking with the reorder point tool, and watch inventory pace via inventory turnover.
Worked example
Avg 40, max 55 → σd = 15. Lead 7 days → √7 ≈ 2.65. At Z 1.65 → SS ≈ 1.65×15×2.65 ≈ 66 units.
Limits and assumptions
σd from max−avg is not a statistical standard deviation. Seasonality and supplier reliability need judgement on top.
Frequently asked questions
What is safety stock?
Safety stock is buffer inventory held above expected lead-time demand so you can still fulfil orders if sales run hot or a delivery is late.
What is the Z factor?
Z is a service-level multiplier. Larger Z means you aim to stock out less often and therefore hold more buffer (for example ~1.65 for a rough 95% service target in a simple normal model).
Why multiply by √lead time?
Uncertainty compounds with longer waits, but not fully linearly — the square-root form is the usual planning simplification.
How does this relate to reorder point?
Reorder point is often average demand during lead time plus safety stock — this page focuses on the safety piece.