To work out a unit price, divide the price by the quantity, then compare products using the same unit: per 100g, per kg, per litre or per item. A unit price calculator does the sums and conversions for you, so you can see at a glance whether the big pack, the multipack or the offer on the small one is genuinely cheaper.
What is a unit price?
A unit price is the cost of a standard amount of a product, such as one kilogram, one litre or one item. It lets you compare packs of different sizes fairly, because the sticker price on its own tells you very little.
UK shelf labels usually show the unit price in small print next to the selling price. The Price Marking Order 2004 sets the units: per kilogram for products sold by weight, per litre for liquids, and per item where products are sold by number. According to government guidance, small shops under 280 square metres do not have to show unit prices for pre-packaged goods in a constant quantity, so you may not see them in every corner shop.
How to calculate price per unit
The formula is simple:
Unit price = total price ÷ quantity
To get a price per 100g, divide the price by the weight in grams and multiply by 100. For price per kg, multiply the price per 100g by 10. The same applies to millilitres and litres.
Quick conversions
| Measure | Equals |
|---|---|
| 1kg | 1,000g |
| 1 litre | 1,000ml |
| Price per 100g × 10 | Price per kg |
| Price per 100ml × 10 | Price per litre |
| 1 UK pint | 568ml |
Mental maths shortcut
If a pack is 250g, multiply the price by four to get the price per kg. £1.10 for 250g is £4.40 per kg, or 44p per 100g. For a 500g pack, double the price.
Worked examples
Cereal: different box sizes
- 500g for £1.50 = 30p per 100g (£3.00 per kg)
- 1.5kg for £3.90 = £2.60 per kg (26p per 100g)
The big box saves 40p on every kilogram.
Washing-up liquid: bottle, big bottle or refill
| Option | Price | Per 100ml |
|---|---|---|
| 450ml bottle | £1.35 | 30p |
| 900ml bottle | £2.25 | 25p |
| 1.5L refill | £3.00 | 20p |
Multipacks: per item
A 9-roll pack of toilet paper at £4.50 is 50p per roll. An 18-roll pack at £8.64 is 48p per roll. The larger pack is only 2p cheaper per roll, so it is worth it only if you have the space and will use them. Check the number of sheets per roll too, as roll sizes vary.
Comparing offers, multibuys and vouchers
Promotions can flip the answer, so always work out the unit price from what you will actually pay.
- Multibuy: two 450ml bottles for £2.40 is £1.20 each, or about 26.7p per 100ml. That beats the 450ml bottle at 30p, but not the 900ml bottle at 25p.
- Voucher or code: an 800g pack at £4.00 with 10% off costs £3.60, or 45p per 100g. A 500g pack at £2.50 is 50p per 100g, so the discounted bigger pack wins. Use our Discount Calculator to get the reduced price first, or the Stacked Discount Calculator if you are combining a sale and a code.
- Loyalty prices: members-only prices count only if you actually have the card.
Shrinkflation and unit prices
Shrinkflation is when a pack gets smaller but the price stays the same. Unit prices expose it. A bag of crisps at £2.00 for 190g works out at about 105p per 100g. If the bag shrinks to 170g at the same £2.00, it becomes about 118p per 100g, a rise of roughly 12%, even though the shelf price has not moved.
If you buy regular staples, jot down the unit price now and then, and compare it later.
When the lowest unit price is not the best buy
- Waste: fresh food you throw away costs more than a smaller pack you finish. Check the use-by date and whether it freezes.
- Storage: bulk buys need cupboard or freezer space.
- Different units: if one label is per 100g and another per kg, convert before comparing. Some items show per item while others show per weight.
- Quality and ingredients: compare like with like. A cheaper product is only better value if it does the job.
- Delivery: shopping online? Delivery fees can erase a bulk saving. See the Free Delivery Calculator to check if a bigger basket is worthwhile.
How to use the calculator
- Enter the price and size of each product.
- Choose the unit: g, kg, ml, litres or items.
- Read the price per unit and see which option is cheapest.
For more ways to cut the cost of your shop, browse the stores on PrimeVoucher for current vouchers and offers.