Unit Price Comparison

The bigger pack is usually cheaper per unit. Usually — and supermarkets rely on you assuming it always is.

The comparison

That is all there is to it, provided both products are measured in the same unit. The traps are not mathematical, they are in the presentation.

In the EU and UK, retailers are legally required to display a unit price alongside the selling price. The requirement is real but the execution is often unhelpful: one product priced per 100 g, the next per kilogram, a third per item. Converting them yourself is frequently the only way to compare.

Where bigger is not cheaper

The assumption that larger packs offer better value is right most of the time and wrong often enough to matter. Documented cases include:

• Promotional small packs. A multi-buy or loyalty price on the small size regularly beats the standard large size.
• “Family” and “value” branding that carries a premium rather than a discount. Consumer groups find this repeatedly.
• Shrinkflation. The pack looks the same and now holds 10 % less. Only the unit price reveals it.
• Different units on the shelf label, making the comparison deliberately awkward.
• Waste. A large pack is poor value if a third of it goes in the bin — effective cost is price ÷ quantity actually used.

Unit price is not the whole story

Three things sit outside the arithmetic and sometimes override it:

1. Consumption effects. A larger pack tends to increase how much you use, which is well documented in food research. The saving per unit can be cancelled by using more units.
2. Cash flow and storage. Bulk buying ties up money and space. Buying a year of washing powder saves 15 % and costs you the use of the money in the meantime.
3. Quality differences. Unit price compares quantity, not what is in it. A cheaper unit price on a product with less active ingredient or more water is not a saving at all — check the concentration.

The honest rule: compare unit prices to avoid being misled, then apply judgement about whether you will actually use it.