To work out a unit price, divide the selling price by the quantity in the standard unit: £3.00 for 500 g is £3.00 ÷ 0.5 kg, so £6.00 per kilogram.
In Great Britain, shops must usually show that figure on the label already. The Price Marking Order 2004, as amended from 6 April 2026, sets out when a unit price is required and how it is calculated. Knowing how to work it out yourself still helps with promotions, loyalty prices, small shops and online baskets, where the comparison is not always on the shelf in front of you.
How to work out the price per kg or per litre
Two steps are enough. First, write the quantity in kilograms, litres or items: 500 g is 0.5 kg, 750 ml is 0.75 litres, a pack of 4 is 4 items. Then divide the price by that number. £1.80 for 750 ml is £1.80 ÷ 0.75 = £2.40 per litre; £0.95 for 200 g is £0.95 ÷ 0.2 = £4.75 per kilogram.
The examples in the table come from the government guidance on the Price Marking Order, published in September 2025, plus two bottles of juice. The cereal shows the point of unit pricing: two boxes at the same £3.00 can differ fourfold in value.
| Product | Selling price | Quantity | Unit price |
|---|---|---|---|
| Cereal A | £3.00 | 500 g | £6.00 per kg |
| Cereal B | £3.00 | 2 kg | £1.50 per kg |
| Apple juice, small | £1.80 | 750 ml | £2.40 per litre |
| Apple juice, large | £2.70 | 1.5 litres | £1.80 per litre |
| Apples, pack of 4 | £3.00 | 4 items | £0.75 each |
| Apples, family pack | £3.00 | 2 kg | £1.50 per kg |
The division is the only hard part. If dividing by 0.75 or 0.2 feels awkward, our guide to long division covers the method, and moving to a round quantity first often makes it disappear: £0.95 for 200 g is £4.75 for five times as much, 1 kg.
Unit pricing in shops: what the law requires
The definition is in article 1 of the Price Marking Order 2004 on legislation.gov.uk, in the version in force since 6 April 2026. The unit price is the final price, including VAT and all other taxes but excluding any deposit, for one item when the product is sold by number, for one kilogram when it is marked only by weight, for one litre when it is marked only by volume, and otherwise for one kilogram, litre, metre, square metre or cubic metre as appropriate.
According to the guidance, the Order covers goods, not services, and sales from traders to consumers, online as well as in physical shops. It applies in Great Britain; Northern Ireland has its own Price Marking Order. A unit price is required for products sold from bulk or required by law to be marked with a quantity, with exceptions: some products listed in Schedule 2, products whose unit price is the same as their selling price, and bread in a prescribed quantity or products pre-packed in a constant quantity when sold in a small shop, by an itinerant trader or from a vending machine. A small shop is one with a relevant floor area of no more than 280 square metres. In an advertisement, the unit price is only required when the selling price is shown.
The same rules apply to online shopping, which the guidance treats like a physical shop. That matters when you compare supermarkets: the unit price shown next to each product online is calculated the same way as the one on the shelf label, so it is a fair basis for comparing baskets between retailers.
Loyalty prices, multibuys and rounding
Since the 2024 amendment, a product sold at more than one price must show each selling price and each unit price. The guidance gives a simple case: a product at £2 for everyone and £1 with a loyalty card must display both prices and both unit prices, together with the condition, here "with loyalty card", clearly and close to the product.
The rules also fix how unit prices are rounded. Below £1, the unit price is shown to the nearest 0.1p: 47.36p becomes 47.4p. Above £1, it may be shown to the nearest penny or to the nearest 0.1p. That is why two shelf labels can show £1.88 per kg and £1.875 per kg for what is, in practice, the same value.
Multibuys deserve a second look. Two packs for £7 instead of £4 each is £3.50 a pack, and the unit price at the offer price is the figure to compare with other brands, not the single-pack one.
Finding the best value without a calculator
When the labels are missing or use different units, bring both products to the same quantity in your head. Comparing a 400 g pack at £1.20 with a 500 g pack at £1.45, think per 100 g: £1.20 ÷ 4 = 30p, and £1.45 ÷ 5 = 29p. The bigger pack is very slightly cheaper.
| Pack | Price | Per 100 g | Per kg |
|---|---|---|---|
| 400 g | £1.20 | 30p | £3.00 |
| 500 g | £1.45 | 29p | £2.90 |
| 1 kg | £2.95 | 29.5p | £2.95 |
| 250 g | £2.10 | 84p | £8.40 |
In this example the 500 g pack is the cheapest per kilogram, even cheaper than the 1 kg pack: bigger is not always better value, which is exactly why unit prices are worth checking.
A price per 100g is a handy halfway house even though the legal unit price is per kilogram: divide the per-kilogram figure by 10, or multiply the per-100 g figure by 10. Halving and doubling do the rest: 250 g at £2.10 is £4.20 for 500 g and £8.40 per kilogram.
These are the same shortcuts as in our guide to mental maths tricks: replace an awkward number with 10 or 100, then halve or double. With a little practice, the comparison takes a few seconds in the aisle.
When the cheaper unit price is not the better buy
A lower unit price saves money only if you use what you buy. A 2 kg bag of potatoes at £1.50 per kilogram is worse value than a 1 kg bag at £1.80 per kilogram if half the big bag goes to waste: you would pay £3.00 for the kilogram you eat.
It helps to put the gap into a percentage. £1.80 per litre against £1.50 per litre is 30p more on £1.50, so 20% more. On a weekly shop, a 20% difference on an item you buy every week adds up; a 2% difference on something you buy twice a year does not. Our percentage calculator works out these gaps if you want to check.
Watch the units, too. If one label gives a price per 100 ml and another a price per litre, convert before comparing: 75p per 100 ml is £7.50 per litre, so a £6.90 litre bottle is cheaper.
Storage, use-by dates and the money you have this week matter too. The unit price is a tool for comparing like with like, not a rule that the biggest pack always wins.
Frequently asked questions
How do you work out the unit price of a product?
Divide the selling price by the quantity in the standard unit. £3.00 for 500 g is £3.00 ÷ 0.5 kg, so £6.00 per kilogram.
Do shops have to show the unit price?
In Great Britain, the Price Marking Order 2004 requires it for products sold from bulk or required by law to be marked with a quantity, with exceptions such as some products in small shops of up to 280 square metres.
Is the unit price the same as the price per 100g?
Not legally: since 6 April 2026, the unit price is defined per kilogram, litre, metre, square metre, cubic metre or item. The price per 100 g is simply the per-kilogram price divided by 10.
How are unit prices rounded?
Below £1, to the nearest 0.1p. Above £1, either to the nearest penny or to the nearest 0.1p.
What happens when a product has a loyalty price?
The shop must show each selling price and each unit price, with the condition for the lower one, such as holding a loyalty card, displayed clearly near the product.
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