Complete Guide on Economic Order Quantity (EOQ)

What Is Economic Order Quantity?

Every buyer faces the same question with every item: order a large quantity once every few months, or small quantities every week? Large orders mean fewer purchase orders and fewer deliveries, but they fill the warehouse and tie up cash. Small orders protect cash, but they multiply admin work and freight and receiving costs. Somewhere between the two is the quantity that makes the combined cost as low as possible. That is the Economic Order Quantity, or EOQ.

The idea is over a century old. Ford W. Harris, an American engineer, published the model in 1913 in an article on how many parts a factory should make at once. It is still one of the most widely used tools in inventory planning because it is simple and gives a sensible starting point for a decision companies make thousands of times a year.

This guide explains the formula and its inputs, shows how to estimate ordering and holding costs from your own data, works through a full example with a cost comparison table, and then covers quantity discounts, the production version of the model, its limits and how it runs inside an ERP.

Two Costs Pulling in Opposite Directions

Ordering cost

Everything the company spends each time it places an order, regardless of size:

• Buyer time to raise and chase the order.
• The approval cycle.
• Fixed freight per delivery, or the carrier's minimum charge.
• Receiving, inspection and data entry at the warehouse.
• Invoice matching and payment processing.

Bigger orders mean fewer orders a year, so annual ordering cost falls.

Holding cost

The cost of keeping one unit in stock for a year:

• Cost of capital: money sitting in stock could repay debt or be invested elsewhere. If inventory is funded by a bank facility, the cost is direct and obvious.
• Storage: rent, power and cooling, security and handling equipment.
• Insurance: fire and theft cover on stock.
• Spoilage and obsolescence: the share of stock that spoils, expires or loses market value.
• Counting and handling: staff time spent on cycle counts and housekeeping.

Bigger orders mean higher average stock, so annual holding cost rises. In the basic model, average stock is half the order quantity, because the balance starts at the full quantity on receipt and runs down towards zero before the next delivery.

The EOQ Formula

EOQ = √(2 × D × S ÷ H)

• D: annual demand in units.
• S: cost per order in SAR.
• H: cost of holding one unit for a year in SAR, usually expressed as a percentage of unit cost, for example 20%.

At this quantity, annual ordering cost equals annual holding cost and their total is at its minimum. A few useful figures follow from it:

• Orders per year = D ÷ EOQ
• Days between orders = working days per year ÷ orders per year
• Total annual ordering and holding cost = (D ÷ Q × S) + (Q ÷ 2 × H)

Estimating S and H From Your Own Data

The hard part is rarely the formula. It is the inputs. Many companies have no idea what one purchase order costs them. Here is a practical, approximate method.

Cost per order

Hypothetically, a purchasing team of two costs SAR 26,000 a month including allowances and social insurance, and spends about 60% of its time on routine purchase orders. The team raises 400 POs a month. Buyer time per order = 26,000 × 60% ÷ 400 = SAR 39. Add warehouse receiving and inspection (say SAR 30), invoice processing in finance (SAR 25) and a fixed delivery charge from the supplier (SAR 100), and the cost per order comes to about SAR 194. Round it to SAR 200.

Holding cost rate

Add up the annual costs tied to inventory and divide by average inventory value. A hypothetical company carrying SAR 3 million of stock on average:

So holding cost is 20% of unit value per year. An item costing SAR 50 costs SAR 10 per unit per year to hold. Use unit cost excluding recoverable VAT, since VAT you reclaim is not a cost to the business.

The rate varies a lot between sectors. Chilled food, pharmaceuticals and fast-ageing electronics run far higher than building materials or metal spare parts, so set a rate per item category rather than one rate for the whole company.

A Full Worked Example

A hypothetical distributor sells an item with annual demand of 12,000 units. Each order costs SAR 200, unit cost is SAR 50 and the holding rate is 20%, so H is SAR 10.

EOQ = √(2 × 12,000 × 200 ÷ 10) = √480,000 ≈ 693 units.

Orders per year = 12,000 ÷ 693 ≈ 17, roughly one every three weeks.

Here is how total cost behaves at different order sizes:

Three things stand out:

1. At EOQ the two costs are almost exactly equal. That is a mathematical property of the model, not a coincidence.
2. The cost curve is flat near the minimum. Ordering 900 instead of 693, 30% more, raises cost by only 3.4%. You can round to a convenient shipping unit, such as a full pallet or master carton, at almost no cost.
3. Big errors are what hurt. Ordering 300 raises cost by 37%; ordering 2,000 raises it by 62%.

The second point should reassure anyone worried about precise inputs. Underestimate ordering cost by 20% and EOQ moves by only about 10%, because of the square root, and total cost moves even less.

How EOQ Differs Between Items

The model gives very different answers depending on the item, which is why one standard order size for everything is a common mistake. Three hypothetical items in the same company, all at a 20% holding rate:

The expensive spare part is ordered in small lots because holding it is costly. The cheap consumable is ordered in bulk because holding a unit costs almost nothing, while ordering often is what adds up. The same logic sits behind ABC classification: high-value items deserve tighter control and smaller, more frequent orders.

EOQ for Imported Goods

Many Saudi companies import by sea, which changes the nature of ordering cost. Each shipment carries large fixed costs: container freight, clearance, port haulage and inspection. Ordering cost is therefore much higher than for local purchases, which pushes EOQ up. Then there is the container constraint. If EOQ equals 70% of a container, it usually makes sense to fill the container with the same item or with other items from the same supplier, because freight will not fall in proportion to the empty space. Decide at shipment level, not item by item, and remember that long lead times also mean more safety stock.

When to Order: Linking EOQ to the Reorder Point

EOQ answers "how much?" The reorder point answers "when?" In the same example, with 360 trading days a year, daily demand is about 33 units. With a 10-day lead time and 150 units of safety stock:

Reorder point = 33 × 10 + 150 ≈ 480 units.

When available stock reaches 480, raise a PO for 693, or 700 after rounding. See our guides to the reorder point and safety stock for the detail.

Quantity Discounts: Should You Order More Than EOQ?

The basic model assumes a fixed price, but suppliers often offer a discount on larger orders. The right decision compares total cost including the purchase price.

Continuing the example, the supplier offers 2% off on orders of at least 1,500 units. Unit price drops to SAR 49 and holding cost to SAR 9.80 (20% of 49).

The discount saves close to SAR 10,000 a year despite the higher holding cost, so taking it makes sense. Before you commit, check three things the formula does not see. Does the warehouse have room for 1,500 units alongside everything else? Does shelf life allow holding the item for about six weeks? Can cash flow handle paying SAR 73,500 at once instead of SAR 34,650?

Economic Production Quantity

In a factory, stock does not arrive all at once from a supplier. It is produced gradually while part of it is being consumed. The adjusted model, Economic Production Quantity (EPQ), replaces ordering cost with machine setup cost and adds the daily production rate (p) and daily usage rate (d):

EPQ = √(2 × D × S ÷ H) × √(p ÷ (p − d))

For the same inputs EPQ is always larger than EOQ, because stock never reaches the full batch size during the run. The practical lesson for manufacturers is that cutting setup time and cost makes smaller, more frequent batches economic, which lowers inventory without raising cost.

Where the Model Breaks Down

• Volatile or seasonal demand: the model assumes steady demand. An item whose sales double in Ramadan needs a different quantity in season.
• Short shelf life: never order more than you can sell before expiry with a safe margin, whatever the formula says.
• Supplier constraints: minimum order quantities, pack sizes and full containers.
• Warehouse capacity: the formula does not know the racks are full.
• New items: no demand history to work from, so start cautiously.
• Very slow movers: an item that sells a handful of units a year is better managed to order.

Treat EOQ as a starting point and adjust for real constraints. Ignore them and you risk stock that stops moving, a problem we cover in our guide to dead stock.

Putting EOQ to Work

1. Start with the items that matter: high annual value and regular demand, where savings are visible.
2. Estimate ordering cost once for the company, using the method above, adjusting freight by supplier.
3. Set holding rates by category: high for chilled goods and electronics, low for durable materials.
4. Calculate, then round to the nearest shipping unit, respecting supplier minimums, shelf life and capacity.
5. Review quarterly: demand and prices change, and the quantity should change with them.

Common Mistakes

• Including recoverable VAT in unit cost, which inflates holding cost and shrinks the quantity.
• Treating capital as free: leaving out financing cost understates holding cost and leads to overstocking.
• Order sizes frozen for years: demand has doubled but the quantity has not, so orders multiply and ordering costs balloon.
• Applying EOQ to everything, including seasonal, custom and slow-moving items where it does not fit.

EOQ Inside an ERP

Ten items in a spreadsheet is manageable. Two thousand items with shifting prices and demand need live data. An ERP supplies the inputs from their real sources: annual demand from issues and sales, unit cost from recent purchase prices and lead time from PO and receipt dates. Once order quantities and reorder points are set, the system proposes purchase orders as thresholds are reached, and buyers review and approve them.

In Tranquil, inventory management handles central inventory and multiple units of measure. That matters for EOQ, because you might calculate in pieces, buy in cartons and store in pallets, and the system converts between them. It connects to purchasing and accounting so every receipt updates quantity and cost immediately.

Frequently Asked Questions

Is EOQ useful for small businesses?

Yes, on items with regular demand. Even rough estimates of ordering and holding cost produce a better quantity than guesswork, because the cost curve is flat around the minimum.

What is the difference between EOQ and the reorder point?

EOQ tells you how much to order; the reorder point tells you when. They work together: when stock hits the reorder point, you order the economic quantity.

How often should EOQ be recalculated?

Quarterly for important items, whenever demand, price or freight cost changes significantly, and before peak seasons.

Should I always take a volume discount?

No. Compare total cost including the purchase price and extra holding cost, then check capacity, shelf life and cash. An attractive discount can end in slow-moving stock.

Does EOQ work for raw materials in manufacturing?

For materials with steady usage, such as packaging and core inputs, yes. Materials tied to specific production orders are better planned from the bill of materials and production schedule, with EOQ used to group requirements into economic orders. Base annual demand on the production plan, not just last year's usage.

Can Tranquil help with these calculations?

Tranquil keeps demand, prices and stock balances in one cloud system linked to purchasing. Contact the Tranquil team to discuss ordering policies for your items.