Inventory management is one of the harder parts of running a growing business. Order too much and you tie up cash in stock that sits on a shelf, but order too little and you risk running out of your best sellers. Most businesses tend to reorder based on what they need in the moment, which rarely results in the most cost-effective quantity being ordered.
Economic order quantity gives you a more structured way to decide how much stock to order. It works out the order size that keeps your total inventory costs as low as possible, balancing the cost of ordering stock against the cost of holding it.
What Economic Order Quantity (EOQ) Means
Economic order quantity (EOQ) is the order quantity that keeps your total inventory costs as low as possible. It does this by balancing two opposing costs: the cost of placing orders and the cost of holding stock. EOQ stands for economic order quantity, and the two terms are used interchangeably.
The model was initially created by Ford W. Harris in 1913 and later developed further by R.H. Wilson, which is why you will sometimes see it called the Wilson formula. Over a century later, it remains a staple of inventory planning because the core trade-off it solves has not changed.
One point that’s worth being clear about from the start: EOQ tells you how much to order, not when to order. The timing question is handled separately by your reorder point, EOQ simply finds the order size where your costs are lowest.
The Economic Order Quantity Formula
The standard EOQ formula is:
EOQ = √(2DS / H)
The three variables are:
- D, annual demand: the total number of units you sell in a year.
- S, cost per order: the fixed cost of placing a single order, including processing, shipping, handling, and admin. This is not the cost of the goods themselves.
- H, holding cost per unit per year: the cost of keeping one unit in storage for a year, including storage space, capital, insurance, and depreciation.
The logic behind the formula is a trade-off. Larger orders mean you place fewer of them, so your ordering costs fall, at the same time, larger orders mean more stock sitting in the warehouse, so your holding costs rise. EOQ is the sweet spot where these two costs balance and the total is at its lowest.
A Worked EOQ Example
For example, say you sell a homeware product with steady year-round demand. After checking your figures, you find:
- Annual demand (D) is 15,000 units
- Cost per order (S) is £40
- Holding cost (H) is £1.50 per unit per year
Putting these into the formula:
EOQ = √((2 × 15,000 × 40) / 1.50)
EOQ = √(1,200,000 / 1.50)
EOQ = √800,000
EOQ = 894 units per order
So the most cost-effective order size for this product is roughly 894 units. With an annual demand of 15,000 units, that works out at around 17 orders across the year. Anything much larger or smaller than 894 units pushes your total cost up. For eCommerce businesses carrying physical stock, calculations like this can help balance cash flow against storage costs, making it easier to avoid both overstocking and unnecessary reordering.
Why EOQ Matters for Inventory Management
Calculating EOQ and using it to guide your ordering brings a few practical benefits:
- Lower total inventory costs: ordering the right amount each time helps you avoid over-ordering, cuts unnecessary storage costs and reduces the risk of damage or stock that never sells.
- Better cash flow: holding only the stock you need frees up working capital that would otherwise sit idle in inventory. You can put that cash towards marketing, product development, or other parts of the business. It can also contribute to a healthier inventory turnover ratio, helping you understand how efficiently stock is moving through your business.
- Fewer stockouts and less overstock: EOQ ties your order size to actual sales, so you are less likely to run out of popular items or end up sitting on stock that ages and loses value.
- Decisions based on data, not guesswork: EOQ uses your real demand and cost figures, which gives you a defensible number to plan around rather than a hunch.
What Ordering and Holding Costs Actually Include
The two costs that trip people up most are the cost per order and the holding cost, because both are made up of more than what is obvious at first glance, but getting them right is what makes your EOQ reliable.
| Holding costs (H) | Ordering costs (S) |
| Capital cost: money tied up in stock | Purchase order creation and processing |
| Storage and space: warehousing or 3PL space and utilities | Receiving and checking goods inbound |
| Service costs: insurance and inventory admin | Supplier communication and order tracking |
| Risk costs: shrinkage, obsolescence, write-offs | Invoice handling and payment processing |
Holding costs are often underestimated because some of them are not billed directly. For example, the capital tied up in stock has a cost even when you have financed it yourself, and space costs apply whether you run your own warehouse or use a third-party logistics provider. When you account for storage and warehousing costs properly, your holding cost figure tends to be higher than a first guess, which changes the EOQ result.
Ordering costs get overlooked for the opposite reason. Placing an order feels like a routine task, so the time and admin behind it rarely gets counted. In practice, it costs more to place ten separate orders for five items each than to place one order for fifty.
The Limits of The EOQ Model
EOQ is useful, but it rests on a set of assumptions that aren’t always true in the real world:
- Demand is constant and known throughout the year
- Ordering and holding costs stay stable regardless of order size
- Lead times are fixed and reliable
- There are no quantity discounts and no minimum order quantities
When those assumptions slip, the result becomes less accurate.
Here are a few common issues worth knowing about:
Demand Rarely Stays Flat
The formula assumes demand stays relatively steady, but in reality, demand rarely behaves itself. Promotions, seasonal peaks, changing customer behaviour and wider market trends can all affect how quickly stock moves. If your EOQ is based on outdated demand data, you could end up ordering too little or tying up more cash in stock than necessary. Reviewing your figures regularly helps keep the calculation accurate and up-to-date.
Supplier Costs Change
Costs have a habit of changing when you least expect them to. Supplier prices rise, shipping costs fluctuate, and operational costs shift over time. An EOQ calculation is only as accurate as the information behind it, which is why it makes sense to revisit it whenever costs change significantly.
EOQ Works One Product at a Time
EOQ is calculated on a product-by-product basis. That works well when you’re managing a handful of items, but it becomes much more time-consuming as your catalogue grows. For businesses managing hundreds or thousands of SKUs, manually maintaining EOQ calculations can quickly become unrealistic.
It Cannot See External Shocks
In practice, EOQ works best as a guide rather than a rulebook. It gives you a strong starting point for making inventory decisions, but it should be reviewed regularly as demand patterns, supplier costs and operating expenses change.
There is also the question of seasonal stock. When demand follows a strong seasonal pattern, a fixed EOQ tends to misfire, leaving you reordering too often in peak and overstocked in the quiet months. For seasonal items, many planners use the Economic Order Interval (EOI) instead, which divides the EOQ by total seasonal demand to set a sensible order cycle.
Putting EOQ Into Practice
If you want to base your ordering on EOQ, a clear process keeps it manageable:
- Gather accurate data: pull together at least a year of demand history along with your current order and holding costs. The output is only as good as the input, so reliable figures matter.
- Calculate EOQ per product: run the formula for each item, starting with your highest-value or fastest-moving lines.
- Compare and apply: set your EOQ figures against your current order quantities and look for the biggest gaps. Apply EOQ where it fits, and add safety stock and reorder points for products with variable demand.
- Recalculate regularly: review your EOQ as demand and costs change. Quarterly is a sensible default for most businesses, with more frequent checks for volatile or seasonal lines.
Once you’re managing hundreds or thousands of SKUs, calculating EOQ manually quickly becomes unrealistic. Inventory management software can automate much of the process, helping you keep calculations up to date as demand and costs change. A fulfilment partner with access to real-time inventory and order data can also make it easier to spot trends and keep your planning accurate.
Used well, EOQ takes some of the guesswork out of reordering and frees up cash you can put back into growing the business.
Understanding economic order quantity is an important part of effective inventory management. But while the formula itself is relatively straightforward, maintaining accurate EOQ calculations can become time-consuming, particularly for businesses managing large product ranges or changing demand patterns.
Green Fulfilment can help you stay on top of inventory performance with accurate reporting and real-time stock visibility. With the right data and processes in place, you can make more informed ordering decisions, reduce unnecessary costs, and keep your operations running smoothly.
Frequently Asked Questions
How do you calculate EOQ?
You calculate EOQ with the formula EOQ = √(2DS / H), where D is annual demand, S is the cost per order, and H is the holding cost per unit per year. For example, with annual demand of 15,000 units, an order cost of £40, and a holding cost of £1.50 per unit, the EOQ works out at roughly 894 units per order.
Remember that EOQ tells you how much stock to order, not when to place the order. To determine the right time to reorder, you’ll also need to calculate your reorder level.
What is the difference between MOQ and EOQ?
Minimum order quantity (MOQ) is the smallest amount a supplier will let you order, so it is set by the supplier and outside your control. Economic order quantity (EOQ) is the order size that minimises your own costs. The two can clash: if a supplier’s MOQ is higher than your EOQ, you may be forced to order more than is ideal.
What is an example of economic order quantity?
If a business sells 15,000 units a year, pays £40 to place an order, and pays £1.50 to hold each unit for a year, its EOQ is around 894 units. That is the order size where its combined ordering and holding costs are at their lowest.
How often should you recalculate EOQ?
There is no fixed schedule, but quarterly recalculation suits most businesses. Recalculate sooner if you see a significant change in demand, if supplier or shipping costs shift, or if your holding costs rise.
Can EOQ be used for seasonal or perishable products?
EOQ works best for products with steady, predictable demand. For seasonal stock, the Economic Order Interval (EOI) is often a better fit, as it adjusts order timing to demand. For perishable goods, you will need to factor shelf life and shorter holding periods into your calculations.
