Every shelf label carries a small figure giving the price per unit of weight or volume. It is the least prominent number on the label and the most useful one.
Package sizes are chosen to defeat comparison
Manufacturers rarely package competing products in matching quantities. One brand sells a container of a certain size, a rival sells something close but not equal.
The mismatch means that comparing headline prices compares two different amounts of product, and the cheaper package is often the smaller one.
Unit pricing removes the arithmetic entirely by restating both as a price for the same quantity. That is the whole reason it is required on shelf labels in many markets.
The larger pack is not automatically cheaper
The assumption that buying bigger saves money holds often enough to become a habit, and retailers are aware of the habit.
On promoted lines the medium size is sometimes the lowest unit price, because the large size is priced against the belief rather than against the cost.
Checking the small figure takes a second and catches this reliably, which is why the ordering by unit price frequently surprises people who expected the opposite.
Multibuy offers need the same treatment
Offers stated as three for a fixed price or two for the price of one and a half are difficult to evaluate against a single item at its normal price.
Converting the offer to a unit price makes the comparison direct, and sometimes shows that a competing product at its ordinary price is still cheaper.
It also exposes offers where the single price was raised shortly before the promotion started, since the offer unit price then matches the old ordinary one.
Different units break the comparison
The system fails when comparable products are labelled in different units: one item priced per hundred grams, a similar item per kilogram, a third per item.
Drained weight adds another complication, since a tinned product may be labelled by total contents including liquid rather than by the food itself.
These inconsistencies are the main practical limit on unit pricing, and they cluster in exactly the categories where price differences are largest.
Concentration changes what a unit means
For detergents, cordials, cleaning products and similar goods, the useful measure is cost per use rather than cost per litre.
A concentrated formulation with a high unit price can deliver more washes than a diluted one that appears cheaper on the shelf label.
Dosage information on the back of the pack supplies the missing factor, and for these categories it matters more than the shelf label does.