To find which supermarket pack costs less for the amount you get, compare its unit price with that of a genuinely similar product. Check that both prices use the same basis—such as cost per kilogram, litre or item—and weigh that against the total checkout cost and how much you can use. A larger pack or a sale label alone does not prove better value.
What a unit price tells you
A unit price expresses a product’s cost for a standard quantity. It puts different pack sizes on a shared basis: for example, the price per 100 g of two cereal boxes or the price per litre of two drinks. The lower figure means you pay less for that comparable quantity, assuming the products are sufficiently alike for the comparison to be useful.
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As the UK Competition and Markets Authority puts it, “Comparing unit prices can help you choose the best value product.” Unit price is a comparison aid, not a verdict on quality, suitability or affordability.
How to compare two products
- Choose genuinely similar products. Compare products that serve the same purpose and have relevantly similar type or format. Sharing a measurement does not make unlike items equivalent.
- Find the unit price. Look beside or below the selling price on the shelf label, or on the online product listing. Retailers often show both the pack price and a price per standard quantity.
- Check the basis. Compare weight with weight, volume with volume, or count with count. Follow the unit used for that product category and make sure both figures use the same measure. If one price is per kilogram and the other per 100 g, convert one before comparing.
- Compare the unit costs. When the products and bases are comparable, the lower unit price costs less for the same quantity.
- Consider the pack price and amount you can use. The lower unit price may come with a higher price at checkout or more product than you can use. Storage, waste and your available budget may affect which option suits you.
Read the numbers, not the pack size
A bigger container can cost more per unit than a smaller one. The CMA’s 2015 example compares three orange juices: 600 ml at 95p costs 15.8p per 100 ml; 1 litre at £2 costs 20p per 100 ml; and 1.5 litres at £3.50 costs 23.3p per 100 ml. In that example, the smallest pack has the lowest unit price.
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Another way to see why size alone is unreliable: UK government guidance calculates that cereal priced at £3 for 500 g works out at £6/kg, while £3 for 2 kg works out at £1.50/kg. The total price is the same in this example, but the cost per kilogram differs because the quantities do.
Calculate a unit price when the label does not show one
Use the selling price and package quantity to calculate the cost of a chosen standard amount:
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Price per chosen standard quantity = (selling price ÷ package quantity) × standard quantity
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Use the unit price to check offers and bulk buys
Compare a promotion using its actual offer price and quantity. If the promotional unit price is displayed, check that it uses a comparable basis; otherwise, calculate it from the offer’s price and quantity. A discount, multi-buy or larger pack is not automatically cheaper per unit. Also consider the total amount you must pay and whether you will use what you buy.
Unit pricing can help spot a higher cost per quantity when a product’s amount changes, but the comparison alone does not explain why the quantity changed. In a 2024 qualitative consumer study, the CMA found that around half of the participating shoppers used unit pricing to varying degrees. It described the result as indicative rather than representative, so it should not be treated as a share of all UK shoppers.
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Why the displayed unit may differ
Weight, volume and count are common comparison bases, but the appropriate measure depends on the product category and local display conventions. UK government guidance also mentions measures such as metres, square metres or cubic metres where appropriate. Do not compare across unlike bases—or across different product formats—unless the quantities and purpose allow a meaningful conversion.
Unit-price display rules are not universal. The legal requirements, covered retailers, product exceptions and rules for promotions depend on location. The guidance below is jurisdiction-specific, not legal advice.
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- United Kingdom: Government guidance on the amended Price Marking Order describes unit prices based on the appropriate measure and notes exceptions. It says small shops with a relevant floor area not exceeding 280 square metres are excluded from some display requirements for pre-packaged constant-quantity products; requirements differ for bulk products. The guidance says traders should endeavour to show promotional unit prices where feasible.
- Australia: The ACCC says most supermarkets, large grocery stores and online grocery stores must display unit pricing in stores, online and in advertising, subject to rules for businesses, locations and products.
- Canada: A federal Office of Consumer Affairs page modified on 4 March 2024 said unit pricing was not generally mandatory across the Canadian marketplace outside Quebec at that time, although some retailers used it voluntarily. Check current provincial rules before relying on a legal claim.
Quick checklist at the shelf
- Are these products similar enough for the comparison to matter?
- Are both unit prices based on the same measure?
- Which has the lower cost per comparable quantity?
- Can you afford the pack price, store the quantity and use it before it goes to waste?
- For an offer, are you comparing the actual promotional price and quantity?
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
