Use the How to Compare Unit Prices When Shopping
To compare unit prices, divide the total price by the total quantity. The lower unit price is usually the better value, as long as the products are genuinely comparable.
Unit price = total price ÷ quantity. If one pack costs £3 for 500g, the unit price is £3 ÷ 500g = 0.6p per gram, or £6 per kg.
Comparing two packs?
The unit price comparison calculator works out the cheaper option, saving per unit and percentage difference.
The unit price formula
A unit price turns different pack sizes into one fair comparison. Instead of comparing the headline price, you compare the price for one gram, kilogram, millilitre, litre, item or serving.
Unit price = total price ÷ total quantity
Example:
£4.50 for 750g
£4.50 ÷ 750g = £0.006 per gram
£0.006 × 1,000 = £6.00 per kgThe important part is using the same unit on both products. Do not compare one item per gram with another per kilogram unless you convert them first.
Always compare the same units
Most mistakes happen when two products use different quantities. Convert both products to the same unit before deciding which is cheaper.
| Product type | Common unit | What to watch |
|---|---|---|
| Pasta, rice, cereal | Price per kg or per 100g | Large packs are not always cheaper once offers are included. |
| Milk, oil, drinks | Price per litre or per 100ml | Check whether the smaller bottle is on promotion. |
| Toilet roll, tablets, batteries | Price per item | Pack count matters more than pack size. |
| Meal kits or protein bars | Price per serving | Serving sizes may differ, so check the label. |
Worked unit price examples
500g vs 1kg
Pack A costs £3 for 500g, so it is £6 per kg. Pack B costs £5 for 1kg, so it is £5 per kg. Pack B is cheaper per kg.
2 litres vs 1 litre
£2.50 for 2L is £1.25 per litre. £1.40 for 1L is £1.40 per litre. The 2L bottle is cheaper per litre.
12-pack vs 10-pack
£10 for 12 items is about 83p each. £8 for 10 items is 80p each. The 10-pack is cheaper per item.
Discounted small pack
If a 400g pack drops from £3 to £2.20, it becomes £5.50 per kg. Compare that against the larger pack before assuming bulk is better.
Include discounts before comparing
If one item is on offer, use the discounted price in the unit price calculation. A smaller pack with a strong discount can beat a larger pack that usually looks cheaper.
Original price = £4.00
Discount = 25%
Discounted price = £4.00 × 0.75 = £3.00
Quantity = 600g
Unit price = £3.00 ÷ 600g = 0.5p per gram
Equivalent = £5.00 per kgUse the discount calculator first if you need the sale price, then use that sale price in the unit price comparison.
A quick method in the shop
If you do not want to do exact maths, round the prices and quantities to get a useful estimate. This is often enough to spot obvious poor value.
Convert both items to a common unit, round the price, divide, and compare. For exact receipts, use the calculator afterwards.
- Choose the same unit, such as kg, litre or item.
- Convert both quantities to that unit.
- Divide price by quantity.
- Pick the lower unit price if everything else is comparable.
Rounding and money values
For shopping, round the final unit price sensibly. Price per item is usually useful to the nearest penny. Price per gram may be easier to read as pence per 100g or pounds per kg.
FAQs
How do I calculate unit price?
Divide the total price by the total quantity. For example, £5 for 1kg is £5 per kg. £3 for 500g is £6 per kg.
Is the bigger pack always cheaper?
No. Bigger packs are often cheaper per unit, but promotions, discounts and pack sizes can change the result. Always compare the unit price.
Should I compare price per gram or price per kg?
Either can work, as long as both products use the same unit. For groceries, price per kg is usually easier to read.
How do discounts affect unit price?
Use the discounted price, not the original price. Then divide that discounted price by the quantity.
What if two products have different serving sizes?
Compare price per serving only if the servings are genuinely similar. Otherwise, compare price per gram, litre or item instead.