Compare up to 3 products to find the cheapest per unit
Unit prices should match shelf labels where available. Always check the pack size on the product.
Unit pricing is one of the most practical tools available to UK shoppers, yet many people overlook it entirely. When you stand in a supermarket aisle choosing between a 400g tin and a 1kg bag of the same product, the shelf price alone tells you very little about which is actually better value. Unit pricing strips away the confusion by normalising costs to a standard measure, such as price per kilogram or price per litre, so you can compare products of different sizes on equal terms. In the United Kingdom, the Price Marking Order 2004 (updated in 2015) requires retailers to display unit prices alongside shelf prices for most pre-packaged goods. Supermarkets must show a price per kilogram, per litre, per metre, or per item, depending on the product type. However, the system is not always consistent. One store might show per-100g pricing while another shows per-kg, and own-brand products sometimes use different reference units to branded alternatives. This inconsistency can make it harder to compare across brands and pack sizes without doing the arithmetic yourself. This calculator removes that friction. Enter the price and quantity for up to three products, select the comparison unit you prefer, and instantly see which item delivers the lowest cost per unit. The savings percentage tells you exactly how much more expensive the priciest option is compared to the cheapest. Whether you are comparing breakfast cereals, cleaning products, or bottled drinks, knowing the unit price helps you make informed choices and potentially save hundreds of pounds over a year of weekly shopping.
To use the unit price comparison calculator: 1. Enter the price and quantity for Item 1. Choose the correct unit (grams, kilograms, millilitres, litres, items, or sheets) to match the packaging. 2. Enter the price and quantity for Item 2. This can be a completely different pack size or even a different brand. 3. Optionally, expand the advanced section to add a third product for a three-way comparison. Leave the price at zero to compare only two items. 4. Select the comparison unit from the dropdown. For solid foods, "per kg" or "per 100g" is standard. For liquids, "per litre" or "per 100ml" works best. For things like kitchen roll or nappies, "per item" is the most useful. 5. Review the results. The calculator highlights which item is cheapest per unit and shows the savings percentage. The bar chart provides a visual comparison of all unit prices side by side. Tip: if you are in the supermarket, check the pack label for the net weight or volume. This is usually more accurate than assuming the pack size from its physical dimensions.
The unit price comparison works by converting all quantities to a common base and then calculating the cost per unit. Step 1: Convert to base units. All weights are converted to grams (1 kg = 1000 g), and all volumes are converted to millilitres (1 litre = 1000 ml). Items and sheets remain as-is. Step 2: Calculate unit price. Unit price = (item price / quantity in base units) x comparison multiplier. For "per kg", the multiplier is 1000 (since the base is grams). For "per 100g", it is 100. For "per litre", it is 1000 (since the base is ml). For "per 100ml", it is 100. For "per item", it is 1. Step 3: Find the cheapest. The calculator compares all unit prices and identifies the lowest. Savings percentage = ((most expensive - cheapest) / most expensive) x 100. Example: Item 1 costs GBP 2.50 for 500g, and Item 2 costs GBP 4.00 for 1 kg. Comparing per kg: Item 1 = 2.50 / 500 x 1000 = GBP 5.00/kg. Item 2 = 4.00 / 1000 x 1000 = GBP 4.00/kg. Item 2 is cheaper, saving 20% compared to Item 1.
Unit pricing is particularly valuable for products that come in many different sizes, such as washing-up liquid, rice, pasta, and tinned goods. Research by Which? has found that shoppers who compare unit prices can save 20-30% on their weekly grocery bill without switching to lower-quality products. Be aware of a few common pitfalls. Promotional multi-buy offers (such as "3 for 2") can sometimes make a smaller pack cheaper per unit than a bulk option. Fresh products with short shelf lives may not be worth buying in bulk if you end up discarding unused portions. And different product formulations (concentrated vs. standard washing liquid, for example) may have different effective costs per use even if the per-litre price differs. For those comparisons, our Cost Per Use Calculator provides a more nuanced analysis. The UK Weights and Measures Act 1985 and the subsequent Price Marking Order establish the legal framework for how products must be labelled and priced. If you notice a retailer not displaying unit prices, you can report it to your local Trading Standards office.