A large pack marked “better value” can still cost more for every wash, serving or sandwich. Learning how to compare supermarket unit prices gives you a quick way to cut through bright shelf labels, multibuys and changing pack sizes. It takes seconds, and it can reduce the cost of a weekly shop without asking your household to settle for less.
What a unit price actually tells you
The unit price is the cost of a standard amount of a product. For food, that is usually shown as pence per 100g, per kg, per litre or per 100ml. For items such as toilet roll, washing capsules and bin bags, it may be shown per roll, per wash or per bag.
The main shelf price tells you what leaves your bank account at the till. The unit price tells you how much value you receive for that money. Two jars of coffee may both look reasonably priced, but the larger jar is only cheaper if its price per 100g is lower.
UK supermarkets are generally required to display unit pricing for comparable goods, usually on the shelf-edge label beneath the product. The type can be small, especially in a busy aisle, but it is one of the most useful numbers in the shop.
How to compare supermarket unit prices in four steps
Start by choosing the product you genuinely need. Comparing the price per litre of semi-skimmed milk is useful; comparing semi-skimmed milk with oat drink is less useful unless you are happy to switch. Unit prices work best when you are comparing like for like.
First, look at the unit written on each shelf label. Make sure the products use the same measurement. One pack may be priced per 100g and another per kg. If so, convert one before deciding: £4 per kg is the same as 40p per 100g.
Next, ignore the package size for a moment and compare the lower figure. A cereal at 32p per 100g costs less than one at 38p per 100g, even if the second box has a lower headline price because it contains less cereal.
Then check whether the cheaper unit price fits your budget and routine. A 2kg bag of rice may offer the best value per kilo, but it is not a saving if it goes stale in the cupboard or pushes this week’s shop over budget. Value is what you will use, not simply what you can carry.
Finally, check promotions separately. A yellow ticket, Clubcard-style price or “2 for” offer can change the calculation. Read the conditions, confirm the number of items you must buy, and compare the promotional unit price with the standard alternative beside it.
A quick example in the pasta aisle
Imagine two packs of penne. A 500g pack costs 85p, with a unit price of 17p per 100g. A 1kg pack costs £1.50, or 15p per 100g. The larger pack is better value by unit price and saves 20p overall if you would have bought 1kg anyway.
But suppose the 500g pack is on offer at two for £1.40. Buying two now works out at 14p per 100g, so the offer is cheaper. If you only need one pack, however, paying 85p may be the right choice for this shop. Savings should support your spending plan, not encourage extra spending.
Watch for the shelf-label traps
Unit pricing is designed to make comparison easier, but it is not always perfectly consistent. Supermarkets may use different units across brands or product types, and promotional labels can make the useful detail harder to spot.
Look closely when comparing products sold in different formats. Laundry liquid may be listed per litre, while capsules are listed per wash. That does not mean one is automatically cheaper. A concentrated liquid can produce more washes from a smaller bottle, so price per wash is the more relevant measure.
The same applies to toilet roll. Price per roll is only a starting point because roll length, sheet count and ply can vary. Compare the number of sheets or total metres where this is shown, then decide whether the thickness and quality suit your household. The cheapest roll on paper may not last longest.
Be alert to “was” prices too. A discount may be real, but the best comparison is still against the current unit prices of competing products, not just against a previous price printed on a sign. This keeps your decision focused on what you can buy today for less.
Compare brands, own-label and different pack sizes
Own-label groceries are often lower in unit price, particularly for cupboard staples such as tinned tomatoes, pasta, flour, rice and frozen vegetables. They are worth checking first, but they are not guaranteed to win every time. A branded item on a genuine promotion can undercut it.
A practical approach is to identify the products where brand matters to you and the products where it does not. You might stick with a preferred tea bag or cereal, while choosing the lowest unit-priced own-label version of chopped tomatoes, oats and cleaning basics. This protects both your budget and your household’s preferences.
Pack size needs the same common-sense test. Bigger packs often cost less per unit because the supermarket uses less packaging and handling per gram or litre. Yet a larger pack is only useful when you have space to store it and time to use it before it spoils.
For fresh food, smaller can sometimes be cheaper in real terms. A big bag of salad with a lower price per 100g is poor value if half ends up in the food waste bin. For long-life, frozen and household essentials, bulk buying is more likely to deliver repeat savings.
Use unit prices before and during your shop
The easiest way to make this habit stick is to use it on a few repeat purchases first. Pick the categories that appear in your trolley every week: breakfast items, packed-lunch food, meat or meat alternatives, fruit and vegetables, cleaning products and toiletries. These small decisions add up because you make them so often.
Before leaving home, compare prices for the key items on your list and note the best options. During the shop, use the shelf label as a final check in case prices or promotions have changed. If you are unsure whether an item is fairly priced, scanning its barcode with a comparison tool such as PricePAL Compare can help you check product pricing quickly rather than relying on a label’s marketing message.
It also helps to set a personal benchmark. If you know you usually pay around 20p per 100g for cereal, 15p per 100g for dried pasta or a certain amount per washing load, you will spot an expensive swap immediately. Your benchmark does not need to be exact. It simply gives you a reason to pause before paying more.
When the lowest unit price is not the best choice
Unit price is a powerful comparison tool, not a rule that overrides everything else. Dietary requirements, quality, ethical preferences, cooking time and convenience can all matter. A more expensive ready meal may prevent an even pricier takeaway, while a higher-priced free-from product may be non-negotiable for your household.
The aim is not to buy the cheapest item in every aisle. It is to know when you are paying extra and decide whether that extra is worth it. That is a far stronger position than choosing based on packaging, a large “save” sticker or the first price you see.
Make every shelf label work harder
Unit prices turn supermarket shopping into a clearer calculation: compare the same unit, check the promotion, buy only what you will use, and keep an eye on the products you purchase most often. A few seconds spent reading the small print can protect pounds in your weekly budget.
Next time a larger pack or special offer catches your eye, look below the headline price before it goes in the trolley. The smallest number on the shelf may be the one that helps your grocery budget go further.