Economic Order Quantity: Order Smarter, Not More Often
Every business that holds stock faces the same tension. Order too often, and you pay ordering costs again and again: supplier admin, delivery charges, receiving, checking, and putting stock away. Order too rarely, and you hold large quantities for a long time, tying up cash, warehouse space, and attention, while risking damage, obsolescence, and shrinkage. The economic order quantity, usually shortened to EOQ, is the order size that balances those two costs. It does not promise to eliminate stock problems, but it gives you a rational starting point instead of ordering by habit or by whatever quantity feels round. The Economic Order Quantity Calculator applies the classic formula to your numbers so you can test different costs and demand levels quickly.
This guide explains the formula in plain terms, shows a worked example with checked arithmetic, and explains where the model breaks down, because EOQ is most useful when you know its limits. It is general business information, not financial advice for any particular business.
The two costs that pull in opposite directions
Ordering cost is the cost of placing one order, regardless of its size. It includes the time spent raising the order, supplier charges that do not depend on quantity, delivery cost if it is fixed per order, and the labour of receiving and checking the delivery. If you order ten times a year instead of five, you pay that cost ten times. Holding cost, sometimes called carrying cost, is the cost of keeping one unit in stock for a year. It includes warehouse space, insurance, handling, damage and shrinkage, obsolescence, and the cost of the capital tied up in stock, which could have been used elsewhere. If you double your average stock level, you roughly double your annual holding cost.
These two costs pull in opposite directions as order size changes. Small orders mean low average stock and low holding cost, but many orders and high total ordering cost. Large orders mean few orders and low total ordering cost, but high average stock and high holding cost. Somewhere between those extremes, total cost is at its lowest. That sweet spot is the EOQ. The classic formula finds it from three inputs: annual demand, ordering cost per order, and holding cost per unit per year. The formula is the square root of two times annual demand times ordering cost, divided by holding cost. In symbols, EOQ equals the square root of (2 × D × S) / H, where D is annual demand, S is ordering cost per order, and H is holding cost per unit per year.
A useful insight follows from the formula: EOQ grows with the square root of demand and ordering cost, not in direct proportion. If demand doubles, EOQ grows by about forty-one percent, because the square root of two is about one point four one four. If ordering cost halves, EOQ falls by about twenty-nine percent, because the square root of one half is about zero point seven zero seven. That dampening effect is good news, because it means small errors in your cost estimates do not cause wild swings in the recommended order size. The total cost curve near the optimum is also fairly flat, so ordering near the EOQ, rather than exactly at it, still captures most of the saving. That flatness matters in practice, because supplier pack sizes, pallet quantities, and discount thresholds often force you to round to a practical quantity anyway.
Gathering inputs you can trust
Annual demand should be based on actual usage or sales over a representative period, not on a single good or bad month multiplied by twelve. If demand is seasonal, consider whether a single annual EOQ is still sensible, or whether you need separate planning for peak and off-peak periods. Ordering cost is often underestimated, because the visible delivery charge is only part of it. Include staff time for ordering, approval, receiving, and invoice matching, even if you estimate it roughly. Holding cost is often the hardest input, because the capital cost and obsolescence risk are not on a single invoice. A common approach is to express holding cost as a percentage of the unit cost, often in the range of twenty to thirty percent per year for many businesses, then multiply by the unit cost to get a per-unit figure. If your stock is bulky, perishable, or fashion-sensitive, your true holding cost may be higher, and your EOQ lower, than a generic percentage suggests.
Be honest about what the inputs exclude. The classic EOQ model assumes demand is steady, lead time is known and constant, stockouts do not happen or are handled separately, and there are no quantity discounts. Real businesses violate all of these at times. That does not make EOQ useless; it makes it a baseline. You handle the violations with separate tools: safety stock for variable demand and lead time, reorder points for timing, and discount analysis for bulk offers. EOQ answers "how much per order", not "when to order" or "how much safety stock to hold", and confusing those questions is a common source of poor stock decisions.
Worked example with sample values
Take a sample product with annual demand of two thousand four hundred units, an ordering cost of fifty per order, and a holding cost of six per unit per year. Apply the formula step by step. Two times demand times ordering cost is two multiplied by two thousand four hundred multiplied by fifty, which equals two hundred and forty thousand. Divide by holding cost: two hundred and forty thousand divided by six equals forty thousand. Take the square root: the square root of forty thousand is two hundred, because two hundred multiplied by two hundred equals forty thousand. The EOQ is therefore two hundred units per order. That is a clean result, chosen for clarity, but the method is the same for messier numbers.
From the EOQ, you can derive the number of orders per year and the total cost. Orders per year equal annual demand divided by EOQ, which is two thousand four hundred divided by two hundred, which equals twelve orders per year, or roughly one per month. Annual ordering cost equals orders per year multiplied by ordering cost, which is twelve multiplied by fifty, which equals six hundred. Average stock, ignoring safety stock, is EOQ divided by two, which is one hundred units, because stock falls from two hundred to zero between deliveries in the simple model. Annual holding cost equals average stock multiplied by holding cost, which is one hundred multiplied by six, which equals six hundred. Total relevant cost, ordering plus holding, is six hundred plus six hundred, which equals one thousand two hundred, excluding the purchase cost of the units themselves, which is the same regardless of order size when there are no discounts. Notice that at the optimum, ordering cost equals holding cost. That equality is a useful check: if your two cost totals are far apart, your order size is probably far from the optimum.
Now test a change. Suppose demand rises to four thousand eight hundred units, double the original. Two multiplied by four thousand eight hundred multiplied by fifty equals four hundred and eighty thousand. Divide by six to get eighty thousand. The square root of eighty thousand is about two hundred and eighty-three, because two hundred and eighty-three multiplied by two hundred and eighty-three equals eighty thousand and eighty-nine, close enough after rounding. Orders per year become four thousand eight hundred divided by two hundred and eighty-three, which equals about seventeen orders. The EOQ roughly doubled in demand but grew by only about forty-one percent, exactly as the square-root relationship predicts. Use the calculator to run your own sensitivity tests in the same way, changing one input at a time and watching which input moves the result most.
Common mistakes to avoid
The first mistake is using purchase cost instead of holding cost in the formula. Holding cost is the annual cost of keeping a unit, not its price, although the price helps you estimate holding cost as a percentage. The second mistake is counting variable delivery charges as fixed ordering cost. If delivery cost rises with quantity, it belongs in a different part of the analysis. The third mistake is applying one EOQ to a group of products that share an order. If several items are ordered together from one supplier, the ordering cost should be shared or the group should be analysed together, not treated as if each item paid the full ordering cost alone. The fourth mistake is ignoring pack sizes and minimum orders. An EOQ of two hundred and eighty-three is not useful if the supplier sells in cases of one hundred; round to three hundred and check the cost difference, which will usually be small because the cost curve is flat near the optimum.
The fifth mistake is forgetting lead time and safety stock. EOQ tells you how much to order, but the reorder point tells you when: average daily demand multiplied by lead time in days, plus safety stock for variability. A business that orders the right quantity at the wrong time will still stock out. The sixth mistake is treating EOQ as permanent. Review it when demand, supplier charges, warehouse costs, or interest rates change materially, and at least once a year for important items. A quantity that was optimal last year can be wasteful this year if demand has fallen or storage costs have risen.
Putting EOQ into a simple routine
Start with your most important stocked items, not your entire catalogue. For each one, record demand, ordering cost, holding cost, EOQ, orders per year, and the practical order quantity after rounding to pack sizes. Compare the total cost at your current order size with the total cost at the EOQ to see where the saving actually lies; some items will show large savings, others almost none, and that tells you where to focus. Then set a reorder point for each item, agree who places orders and who approves them, and review the numbers quarterly for fast-moving items. Keep the analysis simple enough that it gets updated, because a sophisticated model that nobody revisits is worse than a simple one that is maintained.
For related planning, the Inventory Reorder Point Calculator helps you work out when to order, and the Percentage Calculator is handy for expressing holding cost as a percentage of unit cost and for checking changes. You can also read the related guide on comparing supplier quotations at /blog/vendor-quotation-comparator-guide/ for a structured way to weigh price against other terms. EOQ will not solve every stock problem, but it replaces guesswork with a transparent trade-off, and that is a solid foundation for better ordering.