What next?
Price per unit is how you compare unlike packs.
How to use this calculator
- Enter the price and size for the first product.
- Enter the price and size for the second.
- See which is better value per unit.
What your result means
Comparing price per unit — per 100g, per litre and so on — cuts through different pack sizes to reveal the genuinely cheaper option, which often isn’t the bigger pack. Just check the products use the same unit.
Why this one is different
Two items are put into the same unit — per 100 g, per litre, per item — and the cheaper one is named along with the gap between them as a percentage. Each side keeps its own size and its own unit, so a 750 ml bottle and a 2 litre carton go in exactly as they are printed, with no arithmetic first.
Find the real best value on the shelf
The cheapest package is not always the best value. Unit pricing strips away pack size by dividing the price by the quantity, giving a price per gram, millilitre or item you can compare directly — the same thing supermarket shelf labels do, when they bother to.
Enter the price and size of two options and this shows the price per unit for each, names the better buy, and tells you how much cheaper it is. Use the same unit for both items for a fair comparison.
Formula
The lower unit price is the better value, regardless of pack size.
Example calculation
£2.50 for 500 g versus £3.60 for 750 g:
Pack B = 3.60 ÷ 750 = £4.80/kg — better value
Frequently asked questions
Do both items need the same unit?＋
For a fair comparison, yes — compare grams with grams, or millilitres with millilitres. If one is in kg and the other in g, convert first with the weight converter.
Is bigger always cheaper per unit?＋
Usually, but not always. "Bulk" packs sometimes cost more per unit than a mid-size one, which is exactly why checking the unit price pays off.
Why is the large pack sometimes more expensive per unit?＋
Because pack pricing follows demand and promotions rather than volume alone. Multibuy offers on small packs regularly beat the family size, which is why the unit price is worth checking every time.
How do I compare products sold in different units?＋
Convert both to the same base, such as price per 100 g or per litre. Shelf labels in the UK are required to show a unit price, but the units chosen are not always comparable between brands.
Does the cheapest unit price always win?＋
Not if the food spoils or you would not use it. The effective cost of the larger pack includes whatever you throw away, which regularly wipes out the saving on fresh items.
How much can unit pricing actually save?＋
On a typical weekly shop, comparing unit prices across a handful of regular items tends to save a few percent, which compounds over a year without changing what you buy.
Related calculators
Assumptions & limitations
Unit price is the fairest single comparison, but it is not the whole decision.
- Both items must be compared in the same unit. Comparing a price per 100 g against a price per item tells you nothing.
- Multi-buy and loyalty prices are only included if you enter the offer price rather than the shelf price.
- The cheapest unit price is only the best value if you will actually use the larger quantity — waste turns a bargain into a loss, especially with fresh food.
- Packaging weights differ. Drained weight, not net weight, is the fair basis for tinned goods.
Sources & references
Unit pricing rules and how they must be displayed in shops:
- The Price Marking Order 2004 — legal requirement to display unit prices
- Which? Consumer Rights — comparing unit prices and multi-buy offers
Comparing across different units
Shelf labels are only comparable when the unit matches, and supermarkets routinely mix them: one product priced per 100 g, its neighbour per kilogram, a third per item. Convert everything to a single unit before judging — per kilogram and per litre are the most useful, because they make the decimal point large enough to read at a glance. Watch for products sold by drained weight versus total weight, and for multipacks whose unit price is quoted per pack rather than per item, which is where the apparent bargain usually hides.
Bigger is not always cheaper
The assumption that the larger pack has the lower unit price is wrong often enough to be worth checking every time — promotions, pack-size changes and premium formats regularly invert it. The other half of the calculation is waste: a larger pack is only cheaper if it gets used. For perishables, multiply the unit price by the share you realistically expect to consume rather than the whole pack, and the smaller size frequently wins.