Economic order quantity: sizing an order against two costs

The two costs being balanced

Ordering cost is what it takes to place and process a replenishment regardless of size: raising and transmitting the order, receiving, inspecting and putting away, and settling the invoice. Holding cost is what it costs to keep a unit for a period: the capital tied up, storage, insurance, handling, and the risk that the stock loses value before it sells. The optimal size rises with annual demand and with ordering cost, and falls as holding cost rises.

The curve is flat, and that matters more than the formula

Because the total cost function is shallow around its minimum, quantities somewhat above or below the calculated optimum cost only marginally more. This is liberating in practice: it means order sizes can be rounded to a pallet, a layer, a full container or a supplier's minimum without meaningful penalty. It also means arguments over the precise holding cost percentage are usually not worth the meeting, since a plausible range yields nearly the same decision.

Where the classic assumptions break

The standard model assumes steady demand, a fixed unit price, immediate replenishment of the full quantity, and no constraint on capital or space. Real conditions violate all of these. Quantity discounts create step changes that must be evaluated separately at each price break. Perishable or fashion goods carry an obsolescence cost that dwarfs storage. Where capital or storage space is rationed, the constrained answer differs from the unconstrained one, sometimes substantially.

Use it as a diagnostic, not a rule

The most productive application is comparative. If the calculated size is far smaller than what is currently ordered, either holding cost is being ignored or supplier minimums are driving the decision. If it is far larger, ordering cost may be inflated by manual process that automation would remove. Treating the gap as a question about the operating model, rather than as an instruction to change a parameter, is where the analysis earns its keep.

Frequently asked questions

Is the model still relevant when ordering is automated?

Yes, but the balance shifts. Automation lowers the administrative part of ordering cost while the physical part — receiving, inspecting, putting away — remains, and transport economics often dominate. The trade-off does not disappear; its inputs simply change weight.

How should quantity discounts be evaluated?

By comparing total cost at each price break, including the extra holding and obsolescence cost of the larger quantity, rather than by accepting the discount on price alone. A discount that requires holding many months of demand is frequently a transfer of the supplier's stock problem onto your balance sheet.

What holding cost percentage should be used?

One that reflects the true cost of capital plus storage, handling, insurance and value loss for that item class, agreed with finance and applied consistently. Because the cost curve is flat, a defensible figure applied uniformly beats a precise figure applied inconsistently.