About this calculator
Is the big bottle really cheaper? Enter the price, quantity and unit for two or three products and the calculator converts them all to the same price per kilogram, litre or item.
You can mix grams and kilograms, or millilitres and litres, but you can't compare weight against volume. Leave the third product empty if you only have two.
Worked examples
Real numbers, worked out by the same calculator. Press “Use these numbers” to try one above.
500 g at £1.20 versus 1 kg at £2.10
- Product 2: price per kg
- £2.10 per kg (cheapest)
- Product 1: price per kg
- £2.40 per kg
- You save with the cheapest
- £0.30 per kg (12.5% less than the dearest)
Product 2 is the best value at £2.10 per kg, which is 12.5% cheaper than product 1 at £2.40.
Show the working
- Product 1: £1.20 ÷ 500 g = £2.40 per kg.
- Product 2: £2.10 ÷ 1 kg = £2.10 per kg.
- Comparing the price per kilogram, litre or item removes the effect of different pack sizes.
Milk: 2 litres £1.55 versus 568 ml £0.65
- Product 1: price per litre
- £0.78 per litre (cheapest)
- Product 2: price per litre
- £1.14 per litre
- You save with the cheapest
- £0.37 per litre (32.276923% less than the dearest)
Product 1 is the best value at £0.78 per litre, which is 32.276923% cheaper than product 2 at £1.14.
Show the working
- Product 1: £1.55 ÷ 2 l = £0.78 per litre.
- Product 2: £0.65 ÷ 568 ml = £1.14 per litre.
- Comparing the price per kilogram, litre or item removes the effect of different pack sizes.
Three packs of loo roll (items)
- Product 2: price per item
- £0.44 per item (cheapest)
- Product 1: price per item
- £0.50 per item
- Product 3: price per item
- £0.48 per item
- You save with the cheapest
- £0.06 per item (12.5% less than the dearest)
Product 2 is the best value at £0.44 per item, which is 12.5% cheaper than product 1 at £0.50.
Show the working
- Product 1: £4.50 ÷ 9 = £0.50 per item.
- Product 2: £7.00 ÷ 16 = £0.44 per item.
- Product 3: £11.50 ÷ 24 = £0.48 per item.
- Comparing the price per kilogram, litre or item removes the effect of different pack sizes.
How it works
Everything is converted to a common base (grams, millilitres or single items), then the price is divided by the quantity and shown per kilogram, litre or item. Comparing on this basis removes the effect of different pack sizes.
When the bigger pack is not cheaper
Bulk buying often saves money, but not always: a multipack on offer can beat the large size, and a brand's 'family size' can cost more per kilo than the standard pack. Always compare the unit price, which shops are required to display on the shelf label.
Beyond price
The cheapest per kilo is only a saving if you use it all. Fresh food that spoils, or more than you can store, can cost more in waste than the unit price saves.
Frequently asked questions
How do I work out which size is better value?
Divide the price by the quantity to get the price per kilogram, litre or item and compare.
Can I compare grams with kilograms?
Yes, the calculator converts them. It will not compare weight with volume.
What is a unit price?
The price per standard unit, such as per kilogram or per litre.
Why does the shelf label differ from my sum?
Shops sometimes use per 100 g. Multiply by 10 to compare per kilogram.
How many products can I compare?
Up to three. Leave the last one blank to compare only two.
Formulas tested against hand-worked answers. Last reviewed 29 September 2026. These calculators do arithmetic only; they are not financial, tax or legal advice.