The supermarket is the last place most people expect to be gamed, and the first place it happens. Package sizes shrink, “value” packs stop being good value, and the shelf edge prints the one number that cuts through all of it — the unit price — in type small enough to feel optional.
The one number that matters
Unit price is cost per standard measure — per ounce, per litre, per sheet, per capsule. It exists precisely because package sizes vary, and it is the only honest basis for comparison. The mathematics of packaging guarantee that your intuition about which size is cheaper is wrong often enough to be expensive: our recent shelf audit of pantry staples found the larger pack was the worse deal in roughly a third of cases.
- Compare per unit, never per package
- The bigger pack is the better deal far less often than assumed
- Watch the measure: price-per-100-ml and price-per-litre are not comparable
The tricks to know
Retail formats exploit three habits. Size changes: a familiar package quietly shrinks while its price holds — unit price catches it instantly. Different measures: competing brands quoting per-gram versus per-ounce to frustrate mental arithmetic — the unit labels are usually harmonised by law even when the promotion cards are not. And the membership gate: bulk warehouses quote member prices per unit that only win against supermarket list prices, not against discounts you were already going to use.
Building the habit
The skill takes one shopping trip to acquire. Pick your ten most-rebought items and read their unit prices once, deliberately. Most shoppers who do this discover two or three surprises — a store brand that is half the name-brand price per unit, a “value” pack that isn't — and those items then run on autopilot for years. That is the compounding of small arithmetic: a few minutes of attention, a small saving on every trip, a meaningful sum across a decade of groceries.