What the unit price is
A shelf edge label carries two prices. The selling price is what you pay for the pack. The unit price is that price expressed against a standard quantity, most often per kilogram, per litre, per 100g or per 100ml depending on the product.
The distinction matters because only one of them can be compared. Two packs of different sizes have selling prices that cannot be set against each other without arithmetic. Their unit prices can be read directly.
Price marking rules in the UK require both to be given, subject to defined exceptions and to rules on which unit applies to which kind of product. The requirement exists specifically to make comparison possible, which is why the unit price is a piece of consumer information rather than a piece of pricing.
Why it is always the smallest type
On almost every shelf label in the country, the unit price is the least prominent element. That is a design outcome rather than a legal one, and it is worth being clear about the mechanism.
A shelf label has to carry a lot: the product description, the selling price, the unit price, promotional wording where applicable, and often a range or tier marker. Prominence is finite. The elements that drive a purchase decision in the direction the retailer prefers get prominence, and comparison information does not.
Rules address legibility and unambiguity, and there has been sustained policy attention to how clearly unit prices are displayed, particularly on promotions. But the underlying tension remains: the unit price is the number that helps the shopper most and the number that helps the display least.
The practical consequence for a shopper is simply to know where to look. Once you know that the small figure in brackets is the comparable one, the label becomes considerably more useful.
| Element | Purpose | Comparable between products |
|---|---|---|
| Selling price | What you pay for this pack | No, unless the packs are identical in size |
| Unit price | Price per standard quantity | Yes, where the units match |
| Promotional wording | Communicating an offer | No |
| Was price or reference price | Framing the current price | No |
| Range or tier marker | Locating the product in the retailer's range | No |
| Product description | Identifying the item | Not a price element |
Structure set out by this newsroom from the published rules named in the sources below. It is not a survey, a measurement or a market study.
Pack size changes and why the unit price catches them
When a pack size changes and the selling price does not, the selling price tells you nothing has happened and the unit price tells you something has. That is the single most valuable property of unit pricing, and it is why the figure matters far beyond simple size comparison.
Pack sizes change for a range of reasons, including cost pressure, supply, format changes, packing line constraints and regulatory or commercial repositioning. There is no requirement to announce a change, and the pack itself often carries no marker.
The unit price is therefore the only routine, standing indicator of the change available at the shelf. A shopper who reads unit prices notices; one who reads selling prices does not.
The same logic applies to changes that go the other way. A larger pack at a proportionally higher price is invisible in the selling price and visible in the unit price.
Where unit pricing stops working
Three situations reduce its usefulness, and knowing them prevents overconfidence.
Different units for similar products. Where one product is unit priced per kilogram and a comparable one per item, or where drained weight and total weight differ, the two figures are not comparable without further work. Rules address which units apply to which categories, but a shopper still needs to notice when the units differ.
Products where quantity is not the relevant variable. Comparing concentrated and ready-to-use forms of the same thing by weight compares the wrong quantity, because the usable output differs. The unit price is exact and the comparison is still wrong.
Multibuy and complex promotions. Where a promotion changes the effective price, the unit price shown may relate to the standard selling price rather than to the promotional one, unless the display addresses it. Promotional unit pricing has been a specific focus of policy attention for exactly this reason.
What unit pricing is competing against
Shelf comparison is a task carried out in seconds, standing up, often with a distraction present, against a display designed by people who do this professionally. Unit pricing is one small piece of information competing with pack design, price framing, promotional flashes, shelf position and range architecture.
This is not a conspiracy, and it does not require anyone to behave badly. It is the predictable result of an environment in which one party has spent a great deal of effort on presentation and the other has ninety seconds and a trolley.
The useful response is not suspicion but habit. A shopper who consistently reads the unit price has neutralised most of the effect of pack size variation, most of the effect of price framing on multipacks, and a good deal of the effect of range architecture. It is a small habit with a disproportionate effect, which is why this publication returns to it repeatedly.
The rules in outline
Price marking rules require the selling price and, where applicable, the unit price to be indicated for products offered to consumers, in sterling, and in a way that is unambiguous, easily identifiable and clearly legible. There are exceptions and specific provisions for particular kinds of product and sale.
Enforcement sits with local authority trading standards services. Separately, pricing practices that mislead consumers fall under general consumer protection law, now principally the Digital Markets, Competition and Consumers Act 2024, with CMA guidance on unfair commercial practices.
The two regimes do different jobs. Price marking rules govern what must be displayed. Consumer protection law governs whether the overall presentation misleads.
Where this stops
This article describes how a shelf label works. It names no retailer, quotes no price and makes no comparison between businesses, because this publication does not do any of those things. It gives no advice about what to buy.
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