The unit price is the price per standard amount, such as per kilogram, per litre or per 100 grams, and it is the fastest way to compare two packs of the same product. A bigger pack is not always the cheaper pack. This guide explains how a unit price works, how to work one out yourself, and how to use it to save money on your weekly shop.
What is a unit price?
It tells you what you pay for the same amount of product, so you can compare different pack sizes fairly. Many UK shops show it on the shelf label for packaged goods, usually in small print near the main price. Online shops often show it next to or under the price. If it is missing, you can work it out in seconds with the steps below.
How to work out the price per kilogram yourself
- Divide the price by the amount in the pack. This gives you the price per gram or millilitre.
- Multiply to get a standard amount, such as per kilogram or per litre.
Example. A 500g pack costs £2.40, which is £4.80 per kg. A 750g pack costs £3.30, which is £4.40 per kg. The bigger pack is cheaper per kilogram, by 40p. If you buy that product every week, the difference adds up to about £20 over a year.
Our saving calculator helps with the percentage side. Once you know both prices per kilogram, you can see exactly how much cheaper one pack is.
When a bigger pack is not cheaper
- A “family size” can cost more per kilogram than two smaller packs that are on offer.
- A multibuy such as “three for £5” is only a saving if the price per item beats the single pack.
- If you throw food away, the lower price per kilogram is not cheaper for you. You paid for food you did not eat.
- Own-brand packs are often cheaper per kilogram than branded packs of the same size.
Check the units match before you compare
One shelf label may show the price per 100g and the label beside it the price per kg. Convert both to the same unit first. The same goes for litres and millilitres, and for items and sheets. A comparison is only fair when the units are the same.
Quick checks you can do in the shop
- Look for the small print on the shelf label first. It is often there already.
- If two labels use different units, convert them to the same one.
- Compare the price per kg or litre, not the headline price or the size of the pack.
- Think about how much you will use before the product expires or goes stale.
- Check offers too. A multibuy can beat the shelf price, or it can hide a worse deal.
A worked example with an offer
Say a 1 litre bottle costs £1.80 and a 2 litre bottle costs £3.20. Per litre, the first is £1.80 and the second is £1.60, so the big bottle is 20p cheaper per litre. Now a shop puts the 1 litre bottle on offer at “2 for £3”. That is £1.50 per litre, which beats the big bottle. Without working out the price per litre you would never know. This is the sort of check that turns a guess into a saving.
Questions about unit prices
Do shops have to show a unit price?
Most UK shops must show unit prices for many packaged products, but there are exceptions. If you cannot see one, ask or work it out.
Is the lowest price per kilogram always the best choice?
Not always. Quality, how much you will use and storage matter too. It is a tool for comparing, not an instruction to buy the biggest pack.
Where can I read official consumer advice?
Citizens Advice has guidance on prices and your rights when you shop.
The short version
Use the unit price, match the units, and only buy what you will use. To check whether a sale is a genuine saving, read our guide on how to spot a fake discount. Prices change, so always check the label in front of you.
