Economic order quantity
The economic order quantity (EOQ) is a foundational inventory management model that calculates the optimal order size for purchasing or producing goods to minimize the total costs associated with inventory, primarily by balancing the trade-off between ordering costs and holding costs.[1][2] Developed by Ford W. Harris in a 1913 article published in Factory, The Magazine of Management, the EOQ model was an early contribution to operations research and supply chain management, though it gained wider recognition decades later through rediscovery and application in industrial engineering.[1][2] The model operates under key assumptions, including constant and known demand rate, fixed ordering costs per order, constant holding costs per unit per time period, instantaneous replenishment without lead times or shortages, and no quantity discounts.[3][1] At its core, the EOQ formula derives from minimizing the total relevant inventory cost function, which includes setup or ordering costs (incurred each time an order is placed) and holding costs (related to storage, capital tied up, and obsolescence for average inventory levels).[3] The standard EOQ formula is Q^* = \sqrt{\frac{2DS}{H}}, where D represents annual demand in units, S is the fixed cost per order, and H is the annual holding cost per unit; this yields the order quantity that equalizes marginal ordering and holding costs.[2][1] In practice, the EOQ model is applied across engineering economics, business operations, and supply chain contexts to optimize inventory for items with steady demand, such as raw materials or maintenance supplies, often resulting in significant cost reductions—for instance, one case study demonstrated a 61% decrease in inventory costs for a manufacturing firm.[2] While extensions address real-world variations like quantity discounts, lead times, or stochastic demand, the basic EOQ remains a benchmark for efficient replenishment policies in deterministic environments.[3]Fundamentals
Definition and Assumptions
The Economic Order Quantity (EOQ) is the ideal order size that minimizes the combined costs of ordering and holding inventory in a continuous review system, where inventory levels are monitored constantly to trigger replenishment when they reach a reorder point. This model serves as a foundational tool in inventory management, balancing the trade-off between frequent small orders, which incur high ordering expenses, and infrequent large orders, which lead to elevated holding costs due to excess stock.[4] The EOQ concept originated with Ford W. Harris, a production engineer, who introduced it in his 1913 paper "How Many Parts to Make at Once," published in Factory: The Magazine of Management. Harris's work laid the groundwork for the model, though it remained relatively obscure for decades. Independently, R.H. Wilson, a management consultant, derived a similar model and extensively applied it in the 1930s, popularizing it through practical implementations and contributing to its formalization within the emerging field of operations research.[5][6] The EOQ model operates under a set of simplifying assumptions that idealize the inventory environment as deterministic, enabling a tractable mathematical solution without stochastic elements:- Demand occurs at a constant, known rate over time, unaffected by external fluctuations.[4]
- Lead time—the duration between placing an order and receiving it—is fixed and known in advance.
- Replenishment is instantaneous, meaning the entire order arrives at once with no production or delivery delays.[4]
- No quantity discounts are offered, so the purchase price per unit remains constant regardless of order size.[4]
- Shortages are not permitted; inventory must meet demand without backorders or stockouts.
- Ordering costs are fixed per order and independent of quantity, while holding costs are constant per unit per time period.[4]
Variables and Notation
The Economic Order Quantity (EOQ) model utilizes a standardized set of variables to represent its key parameters, enabling consistent mathematical formulation across analyses. These variables are defined as follows:- D: the annual demand rate, expressed in units per year, representing the constant rate at which inventory is depleted over time.[7]
- K: the fixed ordering cost per order, measured in dollars per order, encompassing expenses such as administrative processing, transportation, and setup that do not vary with the quantity ordered.[8]
- h: the holding (or carrying) cost per unit per year, in dollars per unit per year, which includes costs for storage, insurance, spoilage, and opportunity costs associated with tied-up capital.[7]
- Q: the order quantity, in units per order, denoting the size of each replenishment batch placed with the supplier.[8]
EOQ Formula Derivation
The economic order quantity (EOQ) model seeks to determine the optimal order size that minimizes the sum of ordering and holding costs in an inventory system. The derivation begins with the formulation of the total relevant cost function, TC(Q), which captures these two primary components as a function of the order quantity Q. The annual ordering cost is given by (D/Q)K, where D represents the annual demand rate and K is the fixed cost per order; this reflects the number of orders placed per year multiplied by the cost per order. The annual holding cost is (Q/2)h, where h is the holding cost per unit per year and Q/2 is the average inventory level under the assumption of instantaneous replenishment and constant demand. Thus, the total relevant cost is TC(Q) = (D/Q)K + (Q/2)h.[9][10] To find the minimizing Q, denoted Q*, the total cost function is differentiated with respect to Q and set to zero. The first derivative is dTC/dQ = -(DK)/Q² + h/2. Setting this equal to zero yields -(DK)/Q² + h/2 = 0, which rearranges to (DK)/Q² = h/2. Solving for Q gives Q² = (2DK)/h, so Q* = √(2DK/h). This closed-form solution provides the EOQ.[10] An alternative perspective on the derivation emphasizes the trade-off between costs: at the optimum, the marginal increase in holding cost from ordering a larger quantity equals the marginal savings in ordering cost from fewer orders. This balance occurs when the annual holding cost (Q/2)h equals the annual ordering cost (D/Q)K, leading to the same condition (Q/2)h = (D/Q)K and thus Q* = √(2DK/h).[9] To confirm that this critical point represents a minimum, the second derivative of the total cost function is examined: d²TC/dQ² = (2DK)/Q³. For Q > 0, this value is positive, indicating that TC(Q) is convex and the solution is indeed a global minimum.[10]Basic Example
To illustrate the application of the economic order quantity (EOQ) model, consider a hypothetical scenario for a retailer managing inventory of a standard product, such as office supplies. The annual demand is 1,000 units (D = 1,000), the fixed cost per order is $50 (K = $50), and the annual holding cost per unit is $2 (h = $2). These parameters represent typical values in basic inventory scenarios where demand is constant and known, ordering costs include administrative and shipping expenses, and holding costs encompass storage, insurance, and opportunity costs.[11] The optimal order quantity Q* is calculated using the EOQ formula: Q^* = \sqrt{\frac{2DK}{h}} Substituting the values: Q^* = \sqrt{\frac{2 \times 1000 \times 50}{2}} = \sqrt{50000} \approx 223.61 This is typically rounded to the nearest integer for practical implementation, yielding Q* = 223 units per order. The number of orders per year is then D / Q* ≈ 1000 / 223 ≈ 4.48, or approximately 4.5 orders annually.[5] The total annual cost (TC) under the EOQ model is the sum of ordering costs (D/Q * K) and holding costs (Q/2 * h), excluding purchase costs which are constant. For Q* = 223:- Ordering cost ≈ 4.48 × $50 = $224
- Holding cost = (223 / 2) × $2 ≈ $223
- TC ≈ $447
- Ordering cost = (1000 / 100) × $50 = $500
- Holding cost = (100 / 2) × $2 = $100
- TC = $600 (33% higher than EOQ)
- Ordering cost ≈ (1000 / 300) × $50 ≈ $167
- Holding cost = (300 / 2) × $2 = $300
- TC ≈ $467 (4% higher than EOQ)