Unit pricing - the small print that cuts your supermarket bill
Supermarket shelf labels carry two prices. The large one is what you pay at the till, and the small one, usually at the edge of the label, is the price per kilogram, per litre or per 100 grams. That small figure is the only number on the shelf that lets you compare two products honestly, because it is the only one that is unaffected by pack size. Retailers are required to show it under the Price Marking Order, and rule changes in recent years have extended that requirement to promotional prices and tightened how consistently units must be displayed.
Why the headline price misleads
Pack sizes are not standardised, and that is not an accident. When one brand sells 750ml and another sells 800ml, the cheaper headline price can easily belong to the worse deal, and comparing them in your head at the shelf is genuinely hard. The unit price removes the arithmetic entirely.
The same mechanism operates within a single brand through shrinkflation. When a pack quietly drops from 200g to 175g while the price holds, the shelf price is unchanged and the unit price rises by more than 14 per cent. The label records what happened even when the packaging does not.
Comparison only works when the units match, which is the one place the system still trips people up. Some products show price per kilogram and others per 100 grams on the same aisle, and a few, particularly loose items and multipacks, show price per item. Check what unit you are reading before concluding that one product is ten times dearer than another.
The offers unit pricing exposes
Multibuy promotions are the most common case where the maths does not go the way the sign implies. Three-for-two on a premium line frequently works out dearer per 100g than the standard pack of a cheaper line, and buying three of something to save on the third is only a saving if you would have bought three anyway.
Larger packs are usually, but not reliably, cheaper per unit. Cereal, washing powder and toilet roll often reverse it, with the family size costing more per unit than the medium one, because pack size is a branding decision as much as a logistics one. This is worth checking every time rather than assuming, and it takes about two seconds once you know where to look.
| What the sign says | What to check | What you often find |
|---|---|---|
| 3 for 2 | Unit price with and without the offer | A cheaper line beats it outright |
| Big pack, better value | Unit price against the medium pack | Reversed on cereal, detergent, tissue |
| Was £4.00, now £2.50 | Unit price against the own-brand equivalent | Discounted brand still dearer per 100g |
| Loyalty card price | Unit price, and whether you need the card | Genuine on staples, marginal on treats |
| 20% extra free | Unit price on the standard pack | Roughly equal, occasionally worse |
| Meal deal or bundle | Unit price of the components separately | Good on lunch, poor on dinner components |
Where the savings actually are
The biggest single lever in a supermarket is the brand tier rather than the promotion. Most categories run four tiers, from the retailer’s value range through standard own-brand and premium own-brand to the branded product, and the unit price gap between the bottom and the top of that ladder is frequently two or three times. For staples where the product is essentially a commodity (flour, tinned tomatoes, bicarbonate of soda, cleaning products), the step down is often invisible in use.
Not every category survives the drop, and pretending otherwise is how people end up throwing food away. Test one tier down on one product at a time rather than switching a whole shop at once. Where the cheaper version works, it works permanently and compounds every week; where it does not, you have lost one item.
The second lever is buying by unit rather than by convenience format. Pre-chopped vegetables, individually portioned snacks and single-serve drinks all carry a substantial premium per unit for packaging and preparation, and the unit price shows exactly how much. That premium is sometimes worth paying, particularly on things that would otherwise be wasted, but it should be a decision rather than a default.
Doing it without standing in the aisle
Online grocery shopping displays unit prices too, and lets you sort by them, which makes a considered comparison much faster than doing it with a trolley in the way. Building a list of the specific own-brand lines that passed your taste test turns the weekly shop into a repeat order with occasional substitutions, and removes most impulse buying in the process.
When the shelf label is missing, ambiguous or in the wrong unit, the unit price calculator will convert two packs to a common basis so you can compare them directly. It handles the awkward cases, such as comparing a price per kilogram against a price per 100 grams, or a multipack against a single.
Summary
Read the small figure, not the large one, and make sure both labels use the same unit before comparing. Multibuys and family packs are not automatically better value, brand tier saves more than promotions do, and convenience formats carry a premium that the unit price makes visible. Try one tier down on one staple a week and keep what works. Browse food and drink offers, or see our guide to when everything else is cheapest, and find more calculators on the tools page.