Safety Stock Calculator

How it works

1. 1Select your target service level (e.g. 95% maps to Z = 1.65) and enter the standard deviation of demand per period.
2. 2Enter the replenishment lead time in the same periods; the tool computes SS = Z · σd · √(lead time).
3. 3The reorder point is calculated as average demand × lead time + safety stock, giving you the inventory level at which to place a new order.

Use cases

• Setting reorder points in an ERP or inventory management system to prevent stockouts.
• Choosing between service levels to balance holding costs against the risk of running out of stock.
• Communicating buffer requirements to supply chain partners during lead-time reviews.

Frequently asked questions

What is safety stock and why do I need it?

Safety stock is extra inventory held as a buffer against unpredictable spikes in demand or delays in supply. Without it, any demand above average during the lead time causes a stockout. For Z = 1.65, σ = 20 units, and a 4-period lead time: SS = 1.65 × 20 × √4 = 66 units.

How does the service level affect safety stock?

Service level is the probability of not running out of stock during a replenishment cycle. Higher service levels require higher Z-scores: 90% → 1.28, 95% → 1.65, 99% → 2.33, 99.9% → 3.09. Moving from 95% to 99.9% roughly doubles the safety stock needed, so cost grows non-linearly.

What does the reorder point tell me?

The reorder point (ROP = average demand × lead time + safety stock) is the on-hand inventory level at which you should place a replenishment order. If inventory falls to this level before the order arrives, safety stock prevents a stockout under normal variation.