Inventory Control: ABC Analysis, Safety Stock, and Reorder Point
Inventory Control: ABC Analysis, Safety Stock, and Reorder Point
Good inventory control keeps stock available without tying up too much cash. Three tools do most of the work: ABC analysis to prioritize items, safety stock to buffer against uncertainty, and the reorder point to time replenishment. This guide covers all three.
Run these analyses on your items with the free Inventory Control Solver.
ABC Analysis: Focus on What Matters
ABC analysis ranks items by annual usage value (annual demand × unit cost) and groups them:
- Class A — ~20% of items, ~80% of value. Tight control, frequent review.
- Class B — moderate value. Routine control.
- Class C — many items, little value. Simple, bulk control.
The idea (the Pareto principle) is to spend your management effort where the money is.
The Reorder Point
The reorder point (ROP) is the stock level that triggers a new order so it arrives before you run out:
With demand of 1,000 units/year over 250 working days and a 5-day lead time:
Safety Stock: Buffering Uncertainty
Demand and lead times vary, so a buffer protects against stockouts. For variable demand with a constant lead time:
If σ_d = 2 units, lead time = 5 days, and you want a 95% service level:
Choosing a Service Level
A higher service level means fewer stockouts but more safety stock (and holding cost). Common targets are 90% (Z=1.28), 95% (Z=1.65), and 99% (Z=2.33). Class A items usually justify a higher service level than Class C.
How It Fits with EOQ
ABC, safety stock, and ROP work alongside the Economic Order Quantity, which sets how much to order. EOQ answers order size; the reorder point answers when to order. Together they define a complete (Q, R) inventory policy.
Common Mistakes
- Treating all items the same — use ABC to prioritize.
- Zero safety stock with variable demand — guarantees frequent stockouts.
- Mismatched time units — demand, lead time, and σ must be consistent.
Try It Yourself
Enter your items into the Inventory Control Solver to run ABC classification, EOQ, safety stock, and reorder-point calculations with step-by-step working. For the order-size formula on a single item, see the EOQ Calculator.
Key Takeaways
- ABC analysis concentrates control on the high-value items.
- Reorder point = demand during lead time + safety stock.
- Safety stock = Z × σ_d × √(lead time); higher service levels need more buffer.
- Combine with EOQ for a full "how much / when" inventory policy.