The cheapest per unit is not the cheapest if it spoils

There is a settled way of talking about waste and unit price. It is worth asking how much of it survives contact with the detail.

The number that actually matters

Unit price measures cost per unit bought, and what you care about is cost per unit actually used, which are the same figure only when nothing is wasted. A large pack bought at a lower unit price but half discarded costs more per portion consumed than a smaller pack used completely. This is why the same shopping advice produces opposite answers for a single person and for a large household.

Studies of household food waste have generally found substantial proportions of purchased food discarded, though estimates vary widely by country and method. The direction of the finding is consistent enough to act on even where the specific figures are uncertain.

Which products are exposed

Fresh produce, dairy, bread and prepared foods carry the highest waste risk because their useful life is short and inflexible. Anything that must be used within days of opening behaves the same way even when the unopened shelf life is long. Products where the pack cannot be resealed effectively are also exposed, since the clock starts at first use.

Dry goods, tinned items and frozen products are far more forgiving and are where bulk buying genuinely pays. Sorting your regular shopping into forgiving and unforgiving categories is the whole of the technique.

Dates and what they mean

Date labels distinguish between safety and quality in most systems, with one type indicating a safety limit and another indicating expected quality. The terminology and the legal meaning differ by country, so the wording on your own packaging is what matters.

Treating a quality date as a safety date produces avoidable waste, and treating a safety date as advisory produces avoidable risk. Where you are unsure about a specific food, national food safety guidance is the right source rather than general advice. Understanding which label is on a product changes how much of it you will actually use.

Extending the window

Freezing at the point of purchase rather than at the point of expiry converts a short window into a long one for many products. Decanting into smaller sealed containers slows deterioration for anything exposed to air after opening.

Storing items where they are visible has a measurable effect, since most household waste is food that was forgotten rather than rejected. Buying the larger pack and immediately dividing it is often the correct answer rather than buying the smaller one.

These are storage decisions rather than shopping decisions, and they change which pack size is rational.

Multibuys and the same problem

Multibuy offers on perishables are the clearest case where an advertised saving can produce a higher cost per portion consumed. The offer is genuine and the arithmetic is only favourable if the additional quantity is actually eaten.

Run the arithmetic and where the offer is on something freezable, the calculation is straightforwardly good. Where it is on something with a short life and no storage route, it usually is not. The question to ask is not whether the offer is good but whether the extra quantity has a plan attached.

A practical routine

Estimate your household rate of use for the things you buy regularly, roughly rather than precisely. Buy the largest pack you will finish within its useful life, and no larger, whatever the unit price says.

Tracked over a quarter, check what is already in the cupboard before shopping, since duplicate purchases are the second largest source of waste. Plan a use for anything bought on offer at the moment you buy it rather than afterwards. The saving is in the consumption rather than in the purchase.

The takeaway

Buy the largest pack you will actually finish, and decide what happens to the extra before it reaches the cupboard.

A discount is a claim about a price you were never asked to pay.