Reorder point calculation: formula and a worked example.
Order too early and cash sits dead on the shelf. Order too late and the line stops. The reorder point (ROP) is the stock level that triggers a new purchase order at exactly the right moment. Here is the formula, the safety-stock logic, and a full numeric example from a machine shop.
What is a reorder point?
The reorder point is the inventory level at which you place a replenishment order. When on-hand stock falls to the ROP, you order — and the quantity you order is the economic order quantity (EOQ). EOQ answers how much to order; ROP answers when. Both are needed to run a store without stockouts or overstock.
The ROP has to cover the demand that occurs during the lead time — the gap between raising the PO and the material arriving on your shelf — plus a buffer for the days demand or lead time run higher than average. That buffer is the safety stock.
The reorder point formula
The core formula is simple:
If demand and lead time were perfectly constant, safety stock would be zero and ROP would just equal lead-time demand. In the real world both vary, so the safety-stock term is what keeps the line running.
Calculating safety stock
The most common statistical method sizes safety stock from demand variability and a service level:
The Z factor is a business decision: a 95% service level (Z = 1.65) means you accept a stockout risk of about 5% per replenishment cycle. Critical A-class items usually justify 98–99%; low-value C-class items can sit at 90%.
| Service level | Z factor | Typical use |
|---|---|---|
| 90% | 1.28 | Low-value, non-critical items |
| 95% | 1.65 | Standard production materials |
| 98% | 2.05 | Important components |
| 99% | 2.33 | Critical, long-lead, or single-source parts |
Worked example: a CNC job shop
A Pune machine shop consumes a particular bar stock steadily. From twelve months of issue data:
- Average daily demand = 40 bars/day
- Standard deviation of daily demand (σd) = 8 bars/day
- Supplier lead time = 9 days
- Target service level = 95% (Z = 1.65)
Step 1 — lead-time demand: 40 × 9 = 360 bars.
Step 2 — safety stock: 1.65 × 8 × √9 = 1.65 × 8 × 3 = 39.6, round up to 40 bars.
Step 3 — reorder point: 360 + 40 = 400 bars.
So when on-hand stock drops to 400 bars, the planner raises a PO for the EOQ quantity. The 40-bar buffer absorbs the days demand spikes above 40 or the supplier runs a day late.
How ROP works with EOQ
ROP and EOQ are two halves of a continuous-review inventory system. Picture the sawtooth stock graph: stock falls as you consume it, hits the ROP line, you order EOQ, and after the lead time the stock jumps back up by the EOQ amount. Set both correctly and the shelf never runs dry and never bulges. Use the EOQ calculator to fix your order quantity, then apply the ROP formula above to fix the trigger level.
Common mistakes
- Using monthly demand with daily lead time. Keep the time units consistent — daily demand with lead time in days.
- Zero safety stock. Only valid when demand and lead time are genuinely constant, which is almost never.
- Static ROP. Demand drifts with seasonality and product mix. Recalculate quarterly.
- One service level for everything. Segment by ABC class; you cannot afford 99% on every C-part.