Unit Price Comparator

Pack size defeats intuition

Comparing a 750 g box at 4.20 against a 1.2 kg box at 6.30 is arithmetic most people will not do while shopping, so they use a heuristic: the bigger pack is cheaper per unit. That heuristic is wrong often enough to matter. Studies of supermarket pricing routinely find larger packs priced above the smaller equivalent, and promotional pricing on small sizes frequently inverts the expected order.

Normalise the unit first

The comparison only works when both sides use the same unit. Grams against kilograms, millilitres against litres, or count against weight all need converting before dividing. Where one product is sold by weight and a competitor by count, the comparison needs a per-item weight to be meaningful at all, and that is frequently not on the label.

Displayed unit prices are not always comparable

Many jurisdictions require a unit price on the shelf label, which helps, but retailers sometimes use different bases for similar products: per 100 g on one and per kilogram on the next, or per item against per 100 ml. The numbers sit side by side and cannot be compared directly, which defeats the purpose of the regulation.

Drained weight versus net weight

Tinned goods in brine or syrup declare both a net weight and a drained weight, and the drained figure is what you eat. Comparing tins on net weight rewards the one with more liquid. The same applies to glazed frozen fish, where the ice glaze can be a substantial share of the declared weight.

Cheaper per unit is not always the better buy

A larger pack is only cheaper if you use it. For perishables, the waste from an unfinished large pack easily exceeds the unit-price saving, and household food waste figures suggest this is common rather than marginal. Factor in whether the product will actually be consumed before it spoils.

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