Economic Order Quantity Calculator – Optimal Inventory EOQ

EOQ is the ideal order quantity a company should purchase to minimize total inventory costs—including ordering costs (shipping, handling, administrative) and holding costs (storage, insurance, obsolescence). Developed by Ford W. Harris in 1913 and later refined by R.H. Wilson, the EOQ model finds the sweet spot where these two opposing costs balance each other out. The formula is: EOQ = √(2DS/H), where D is annual demand, S is ordering cost per order, and H is annual holding cost per unit.

The classic EOQ model assumes: (1) Constant demand — demand is known and uniform throughout the year; (2) Fixed ordering cost — each order costs the same regardless of quantity; (3) Fixed holding cost — cost per unit held is constant; (4) Instantaneous replenishment — orders arrive all at once; (5) No stockouts — shortages are not allowed; (6) No quantity discounts — unit price is constant regardless of order size. While real-world scenarios may deviate, EOQ provides an excellent starting point for inventory optimization.

Holding cost (also called carrying cost) typically includes: Warehouse rent/space costs, Insurance premiums on stored inventory, Cost of capital tied up in inventory (opportunity cost, often 5–15% of unit value), Obsolescence and spoilage, Inventory tracking and management labor, and Taxes on stored goods. In practice, annual holding costs often range from 10% to 30% of the unit cost. When using the percentage method in this calculator, a rate of 10–25% is common.

Ordering cost includes all expenses incurred each time you place an order: Shipping and freight charges, Administrative processing (purchase orders, approvals, data entry), Receiving and inspection labor, Supplier communication costs, and Payment processing fees. To estimate S, divide your purchasing department's annual operating cost by the number of orders placed per year. Typical values range from $25 to $200+ per order depending on business complexity.

A key insight of the EOQ model is that the total cost curve is relatively flat (insensitive) around the optimal point. This means you can deviate from the exact EOQ by 10–20% and incur only a small increase in total cost (often less than 2%). This property makes EOQ robust in practice—you don't need extreme precision. For example, ordering in convenient round numbers or adjusting for package sizes won't significantly impact your bottom line. This "flat bottom" characteristic is why EOQ remains practical despite its simplifying assumptions.

The basic EOQ model assumes a constant unit price. When suppliers offer quantity discounts (lower price for larger orders), the analysis becomes more complex. You must compare the total annual cost (including purchase cost) at each price break. Sometimes ordering more than the basic EOQ to get a discount results in lower overall costs—even though holding costs increase. This is called the EOQ with quantity discounts model. Our calculator uses the standard EOQ; for discount scenarios, calculate total cost at each price tier manually.

The Reorder Point (ROP) is the inventory level at which you should place a new order to avoid stockouts. It's calculated as: ROP = (Daily Demand) × (Lead Time). Daily demand = Annual Demand / Working Days per Year. For example, with 12,000 units/year, 250 working days, and a 5-day lead time: ROP = (12,000/250) × 5 = 240 units. When inventory drops to 240 units, you place an order for the EOQ amount. This assumes constant demand; in practice, you may add safety stock to buffer against variability.

The standard EOQ model optimizes one product at a time. For multiple products from the same supplier, you may benefit from joint ordering—combining items into a single order to save on ordering costs. This requires a modified approach that considers shared ordering costs and potentially different holding costs per item. Additionally, constraints like truckload capacity, budget limits, or warehouse space may require adjusting individual EOQs. For multi-item optimization, consider using inventory management software or a tailored multi-product EOQ model.