Supply Chain Calculators: EOQ, Safety Stock, Reorder Point and Inventory Turnover

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EOQ: balancing ordering cost against holding cost

Economic order quantity answers the most basic inventory question: how much should you buy in one order? Order in large batches and you place fewer orders a year, so ordering cost falls, but average inventory climbs and holding cost rises. Order in small batches and holding cost drops while ordering cost climbs. EOQ is the batch size where those two curves cross and their sum is at its minimum, given by the square root of two times annual demand times ordering cost, divided by the annual holding cost per unit. Because the total-cost curve is flat near the optimum, EOQ is forgiving: a moderate deviation from the exact figure barely raises cost, which is why it remains the anchor of inventory policy decades after Ford Harris derived it.

Safety stock and service level: paying for protection

Safety stock is the reserve that keeps a line or a shelf supplied when demand runs hot or a delivery runs late. Its size is a direct trade between the cost of carrying extra inventory and the cost of a stockout. The service-level method makes that trade explicit: a target cycle service level maps to a Z-score, and safety stock equals that Z times the standard deviation of demand over the lead time. The relationship is nonlinear, so moving from 95 to 99 percent service costs far more buffer than moving from 90 to 95. Sizing safety stock well means choosing a service target that reflects the real cost of a shortage rather than defaulting to a round number.

Reorder point: turning the buffer into a trigger

The reorder point converts the plan into action. It is the on-hand quantity at which a replenishment order should be placed, and it equals expected demand during the lead time plus the safety stock. Set it too low and stock runs out before the order arrives; set it too high and inventory piles up. Because it builds directly on lead-time demand and safety stock, the reorder point ties the ordering and buffering decisions together into a single number the warehouse can act on, which is why kanban systems and min/max policies are both just physical expressions of a reorder point.

Inventory turnover and ABC: measuring and focusing

Turnover and ABC analysis are the diagnostics that tell you whether the policy is working and where to spend attention. Inventory turnover, cost of goods sold divided by average inventory, shows how many times stock cycles in a year; its inverse, days of inventory, shows how long stock sits on average. ABC analysis applies the Pareto principle to inventory: a small share of items usually accounts for most of the annual value, and sorting items into A, B, and C tiers focuses tight control on the few that matter and loosens it on the many that do not. Together they turn a pile of stock into a ranked, measurable system.

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