To compare supermarket prices fairly, compare the unit price of similar products using the same measure. The lower unit price costs less for a comparable quantity—but also check the amount you pay at checkout, whether you can use the whole pack, and whether the products suit your needs. A larger pack or a sale label alone does not prove better value.
What a unit price tells you
A unit price shows the cost of a standard quantity, such as a kilogram, litre, or individual item. It lets you compare products sold in different pack sizes on a common basis. The UK Competition and Markets Authority puts it simply: “Comparing unit prices can help you choose the best value product.” (CMA, 2015.)
For example, the CMA compared three orange juice packs: 600 ml at 95p works out to 15.8p per 100 ml; 1 litre at £2.00 is 20p per 100 ml; and 1.5 litres at £3.50 is 23.3p per 100 ml. In this example, the smallest pack has the lowest unit price. The total pack price and the cost per comparable quantity answer different questions: what you pay now, and how much that amount costs relative to other options.
How to compare unit prices while shopping
- Choose genuinely similar products. Compare products that meet the same need, taking account of type and format. A shared measurement does not make two products equivalent in quality, composition, or usefulness.
- Find the unit price. Check the shelf label beside or below the pack price, or the product listing online. Note the unit as well as the number—for example, price per 100 g or per litre.
- Match the measurement basis. Compare weight with weight, volume with volume, and count with count. Follow the standard measure used for the product category and the units shown on the labels.
- Compare the unit prices. When the products and measures are comparable, the lower unit price means less cost per equivalent quantity.
- Check the full purchase trade-off. Consider the checkout price, how much you need to buy, whether you can use it before it goes to waste, and whether you have room to store it.
Depending on the product and local display conventions, relevant bases can include price per kilogram or per 100 grams, per litre or per 100 millilitres, or per item. UK guidance also identifies measures such as metres, square metres, and cubic metres for appropriate products. (UK government unit-pricing guidance; Australian Competition and Consumer Commission guidance.)
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Calculate the unit price if it is missing or unclear
Use this formula:
Price per chosen standard quantity = (selling price ÷ package quantity) × standard quantity
Keep the units consistent. To work out the price per 100 g for a 500 g pack, divide the pack price by 500, then multiply by 100. If one label gives a price per kilogram and another per 100 g, convert one so both use the same basis before comparing.
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The UK government illustrates the method with cereal: £3 for 500 g equals £6 per kilogram, while £3 for 2 kg equals £1.50 per kilogram. (UK government unit-pricing guidance.) A basic calculator can help with the arithmetic, but it is optional when a clear unit price is already shown.
Check whether a promotion is actually cheaper
Compare the promotional unit price with the regular unit price, if both are available. If the promotion does not show a unit price, calculate it using the actual offer price and quantity you will receive. Do not assume a multi-buy offer, percentage discount, or “special” label is better without checking the price per comparable quantity.
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Promotional display rules vary by location, and not every offer is required to show a unit price everywhere. UK guidance advises traders to display promotional unit prices wherever feasible, while noting that exemptions apply. (UK government unit-pricing guidance.)
Why the lowest unit price may not be your best choice
- It may cost more at checkout. A larger pack can have a lower price per unit but require a higher upfront payment.
- You may not use the whole amount. A bulk pack is poor value for you if some of it expires or is wasted.
- The products may not be equivalent. Unit prices compare quantities, not quality, ingredients, usable contents, or suitability.
- The format may affect the comparison. If the products use different measures or formats, check whether a meaningful like-for-like comparison is possible rather than treating the numbers as interchangeable.
Unit pricing can also help flag when a product’s amount changes relative to its price. A unit-price comparison shows the cost per quantity; by itself, it does not establish why the quantity changed. (CMA, 2024 consumer study.)
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Unit-price display rules depend on where you shop
Whether a retailer must display unit prices, which products are covered, and which measures or exceptions apply depend on local rules. Do not assume a label is legally required—or that a missing unit price means the product cannot be compared—without checking your jurisdiction’s current guidance.
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- United Kingdom: Government guidance on the amended Price Marking Order describes appropriate measures and exceptions. It says shops with a relevant floor area not exceeding 280 square metres are excluded from some unit-price display requirements for pre-packaged products sold in constant quantities; rules differ for bulk products. The 280-square-metre threshold is specific to those UK exceptions, not a general rule for all retailers or locations. (UK government guidance.)
- Australia: The ACCC says most supermarkets, large grocery stores, and online grocery stores must display unit pricing in-store, online, and in advertising, subject to rules for businesses, locations, and products. (ACCC guidance.)
- Canada: A federal Office of Consumer Affairs page modified on March 4, 2024, said unit pricing was not generally mandatory across the Canadian marketplace outside Quebec at that time, although some retailers used it voluntarily. That is a date-bounded description; check current provincial rules before relying on it. (Office of Consumer Affairs.)
A quick decision check
- Are the products similar enough for a useful comparison?
- Are both prices expressed using the same measure?
- Is the lower unit price worth the total amount you must pay?
- Can you store and use the quantity you are buying?
- For an offer, have you used its actual 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.
