Unit pricing exists so shoppers can compare a large box against a small one without arithmetic. In practice the comparison fails often enough that the label alone is an unreliable guide.
The units are not consistent across a shelf
Regulations require a unit price but allow several bases for it. One product may be priced per ounce, its neighbour per hundred count and a third per item.
Paper goods are the clearest case, where one label reads per sheet and another per roll, and the rolls contain different numbers of sheets of different sizes.
Where the bases differ, the two figures cannot be compared at all, and the label's apparent precision hides that.
Drained weight changes the comparison
Canned and jarred goods are usually unit priced on total contents including liquid, which is not what gets eaten.
Two cans with the same net weight can hold noticeably different amounts of solid food, and the drained weight is often printed separately in smaller type.
The same problem appears with frozen goods carrying a glaze of ice and with products sold by volume that vary in density.
Promotional prices are not always reflected
Unit price labels are printed from the regular shelf price. When a temporary promotion is applied, the unit figure on the label may not be updated.
Multi-buy offers make this worse, because the unit price shown describes a single item rather than the price achieved by taking the offer.
Loyalty-only prices introduce a third figure, so a single shelf edge can carry a unit price that applies to none of the ways the product is actually bought.
Bigger is not reliably cheaper
The assumption that the large pack wins is often wrong. Promotions concentrate on mid-size packs, and the largest size is frequently the least discounted.
Family sizes also carry their own packaging and handling costs, and in some categories the large format is positioned as a convenience item rather than a value one.
Checking the unit price rather than assuming it is the point of the label, but it only works where the units genuinely match.
Waste turns the arithmetic around
A cheaper unit price on a large pack is only a saving if the contents are used before they spoil.
Fresh produce, dairy and bakery items bought in bulk by small households frequently end up discarded, which raises the effective cost above the smaller pack.
The useful comparison is cost per portion consumed, and no shelf label can calculate that because it depends on the household rather than the product.