Do UK shops have to show a unit price?

For most pre-packaged goods sold by weight or volume, yes — price marking rules require a unit price to be displayed alongside the selling price. It is usually in small print on the shelf edge label.

Is the bigger pack usually better value?

Often, but not reliably enough to assume. Promotions on smaller sizes regularly invert it, and the only way to know on any given day is to read both unit prices.

Does unit price work for non-food items?

Yes, wherever a sensible unit exists — per sheet for kitchen roll, per wash for detergent, per tablet for medicine. The trap is comparing items that use different units.

Unit price: the only honest way to compare

Two products, two pack sizes, two prices. There is only one number that makes them comparable.

Updated 2 September 2026 · 2 min read

Stand in front of any supermarket shelf and you are being asked to solve a small arithmetic problem under time pressure, holding a basket, possibly with someone waiting behind you. One box is larger and costs more. Another is on a multibuy. A third is a different brand in a different shape entirely.

Almost nobody does the maths. The shelf is arranged on that assumption.

The number that solves it

The unit price — the cost per 100g, per litre, per sheet, per wash — collapses every difference in pack size into a single comparable figure. In the UK it is legally required to be displayed for most pre-packaged goods, which is why it is there on the shelf label, usually in the smallest type on the ticket.

It is the only number on the label that lets you compare two different products honestly, and it is printed smaller than everything else. That placement is not an accident.

Why the big pack is not always cheaper

The assumption that buying larger saves money is so widely held that it is now something retailers can price against. Larger packs frequently do offer a lower unit price — but not reliably, and the exceptions cluster in predictable places:

When the small pack is the one on promotion. A discount on the small size regularly takes it below the large one, and the shelf does not announce this.
When the large size is the "family" or "value" pack. These names are marketing positions, not pricing commitments.
When the small size is the competitive one. Shops price keenly on the sizes shoppers compare across brands, and less keenly on the ones they do not.
When you will not use it all. A better unit price on food you throw away is a worse price on food you eat.

When the unit prices are not comparable

The unit price only works if both products are measured the same way, and this is where it quietly breaks:

Different units for the same thing. Per 100g against per item. Per wash against per litre. If the units differ, you are not comparing.
Drained versus total weight. Tinned goods can be priced on either. A tin priced on total weight looks better and gives you less food.
Concentrates. A concentrated liquid at a higher price per litre may be cheaper per use. Per-wash or per-dose is the honest unit here.
Different products wearing the same name. Two things called the same thing can differ in what is actually in them. The unit price compares quantity, never quality.

The habit worth building

You do not need to do this for a whole shop. Do it once, properly, for the ten or fifteen things you buy every week — the staples, the cleaning products, the pet food. Those are the purchases that repeat, and a repeated purchase is where a small difference compounds into a real one.

Everything else is a one-off, and a one-off is worth less of your attention than the shelf layout would suggest.