Safety stock and reorder points

A reorder point is a trigger for replenishment. It combines the demand expected while new supply is on the way with a buffer for uncertainty. The buffer is a policy choice backed by evidence—not a universal number to copy across products.

1. Begin with the coverage window

In a simple continuous-review policy, the reorder point (ROP) covers expected demand during replenishment lead time plus safety stock:

ROP = average demand per day × replenishment lead time in days + safety stock

For example, if demand averages 12 units a day, replenishment takes 5 days, and the selected buffer is 18 units, then ROP = 12 × 5 + 18 = 78 units. If the inventory position is 52 units, a simple order-up-to-the-trigger calculation suggests 26 more units.

Inventory position is often calculated as usable on-hand plus qualifying open orders minus allocations or commitments. Count a receipt only if it is expected before the stock is needed.

2. What the buffer is protecting against

Safety stock can protect against demand variation, lead-time variation, yield loss or a combination. The right model depends on which source of uncertainty matters and what data are available. If daily demand is variable but lead time is treated as fixed, a common approximation is:

safety stock ≈ z × daily demand standard deviation × √(lead-time days)

This approximation assumes independent daily demand with stable variance, a fixed lead time, and a normal approximation for demand over the lead-time window. For a 95% one-sided cycle-service target, z is about 1.645. With a daily standard deviation of 4 units and 5 days of lead time, the estimated buffer is about 15 units. That gives an illustrative ROP of 75 units (60 expected demand + 15 buffer).

When lead time also varies, demand is intermittent, or the review cycle is periodic, this formula is not enough as written. A periodic-review policy must cover the review interval as well as lead time. Use a model that matches the actual policy and data.

3. Do not confuse service measures

Cycle service level is the chance of avoiding a stockout during a replenishment cycle under a specified model. Fill rate is the share of demand supplied immediately. They answer different questions; the same buffer can produce different results under different demand patterns and shortage rules.

4. Review buffers by segment

• Check demand history for stockouts, returns, promotions and product transitions.
• Use actual supplier lead-time history, not only a fixed master-data value.
• Segment products by variability, criticality, margin, expiry and substitution options.
• Review the buffer when demand, supply, service policy or product lifecycle changes.

A bigger buffer can protect service, but it also ties up cash and may age into obsolete stock. Review the cost of both overage and shortage before changing the target.