Economic Order Quantity, Explained for People Who Actually Place Orders
If you order too little too often, you drown in purchase orders, freight charges, and receiving paperwork. If you order too much at once, cash sits frozen on a shelf, sometimes spoiling or going obsolete. Economic Order Quantity (EOQ) is a simple model that finds the sweet spot between those two costs. After a year in which many organizations learned the hard way what running out of stock feels like, it is worth understanding the basic math before you over-correct and stockpile everything.
The two costs EOQ balances
Every reorder decision pits two costs against each other. Ordering cost is what you pay each time you place an order regardless of size: the buyer's time, supplier setup, inspection, and the administrative overhead of a purchase order. Holding cost is what it costs to keep one unit in storage for a year: warehouse space, insurance, capital tied up, shrinkage, and obsolescence. Order in large batches and your ordering cost per unit drops but your holding cost climbs. Order in small batches and the reverse happens. EOQ finds the quantity where the total of the two is lowest.
The formula, and what each input means
The classic EOQ formula is the square root of (2 × annual demand × cost per order) divided by (annual holding cost per unit). Suppose you use 12,000 units a year, each order costs $50 to place, and holding one unit for a year costs $3. That gives the square root of (2 × 12,000 × 50) / 3, which is the square root of 400,000, or roughly 632 units per order. You would place about 19 orders a year. Notice the inputs you need:
Annual demand. How many units you actually consume in a year — use real usage history, not a hopeful forecast.
Cost per order. The fully loaded cost of placing one order, including people's time, not just the freight line.
Holding cost per unit. Often estimated as a percentage of unit value (commonly 15–30%), covering storage, capital, and risk of obsolescence.
Where the textbook model breaks down
EOQ assumes steady, predictable demand, a constant price per unit, and instant replenishment with no shortages. The pandemic recovery exposed how rarely those assumptions hold. Use EOQ as a starting estimate, then adjust for reality.
Quantity discounts: if a supplier cuts the price at a higher volume, compare the total cost at the EOQ against the total cost at each discount break — the discount often wins.
Volatile or seasonal demand: EOQ uses an average, so it smooths over spikes. Pair it with a safety stock and a sensible reorder point.
Long or unreliable lead times: EOQ tells you how much to order, not when. With disrupted shipping, the reorder point matters more than the batch size.
Shelf life and obsolescence: for perishable or fast-changing goods, the holding cost is effectively higher than the spreadsheet shows.
The real value of EOQ is not the precise number it spits out — it is the discipline of naming your ordering and holding costs and deciding deliberately, instead of reordering by gut feel. Run it on your highest-value or highest-volume items first, where a better order size frees up the most cash.
If you want a sharper view of your reorder logic, supplier terms, and total landed cost, XNM's procurement, sourcing & contract management can help you turn the math into practical buying decisions.