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Two packets can look much the same while containing different amounts. A larger bag can cost less per gram but more at the till, while a multipack may contain smaller individual portions than the single packs beside it.
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Learning how to compare unit prices gives you a consistent starting point. Instead of judging only the price on the front of the shelf label, compare what the same quantity costs, then decide whether the product and pack size fit your needs.
Read the unit, not just the shelf price
Find the pack price, the pack quantity and the unit price before deciding which option costs less for the same amount.
The pack price tells you what you pay for the whole product. The unit price expresses that cost for a standard amount, such as 100g, 1kg, one litre or one item.
Read the unit as carefully as the number. A figure shown per 100g is not directly comparable with one shown per kg. Also check that the shelf label matches the exact product, flavour and pack size you are holding.
- Price per 100g: useful for comparing products sold by weight.
- Price per kg: another weight-based comparison; 1kg equals 1,000g.
- Price per litre: useful for comparing equivalent liquids; one litre equals 1,000ml.
- Price per item: useful when the individual items are reasonably comparable in size and purpose.
Match the units before doing the maths
Put both products on the same measurement basis before comparing their unit-price figures.
To convert a price per 100g into a price per kg, multiply it by ten. To convert a price per kg into a price per 100g, divide it by ten. The same approach works between a price per 100ml and a price per litre.
If you need to calculate a unit price yourself, divide the pack price by the quantity, then multiply by the comparison amount. For price per 100g, divide the price by the weight in grams and multiply by 100.
Keep both prices in pounds or both in pence while calculating. Mixing pounds and pence can make a straightforward comparison misleading.
- Weight: compare per 100g with per 100g, or per kg with per kg.
- Volume: compare per litre with per litre, or per 100ml with per 100ml.
- Count: compare per item only after checking the size of each item.
Use a worked example to compare pack sizes
The lowest pack price and the lowest unit price can belong to different products.
These figures are invented solely to demonstrate the calculation. They are not current supermarket prices or advertised offers. Assume the pasta is otherwise equivalent for your purposes.
The 750g pack has the lowest price per 100g in this example. The 500g pack has the lowest total price, while the largest pack has the highest unit price.
Your choice still depends on quantity. If you only want 500g and have no use for the extra, spending more on the 750g pack may not be worthwhile.
| Pack | Hypothetical pack price | Calculation | Price per 100g |
|---|---|---|---|
| 500g pasta | £1.50 | £1.50 divided by 500, multiplied by 100 | 30p |
| 750g pasta | £2.10 | £2.10 divided by 750, multiplied by 100 | 28p |
| 1kg pasta | £3.20 | £3.20 divided by 1,000, multiplied by 100 | 32p |
Compare like with like across brands
Compare equivalent products first, then use your preferences and requirements to decide whether they are genuine alternatives.
Start with products that do the same job for you: plain porridge oats with plain porridge oats, for example, rather than plain oats with a flavoured instant breakfast sachet.
For own-brand and branded products, compare the unit price alongside the pack description, ingredients and format. A lower unit price tells you about quantity for your money; it does not tell you whether you will prefer the taste or find the product suitable.
If you are unsure about an unfamiliar alternative, a smaller pack may be a more useful first purchase than a large quantity chosen only for its unit price.
Check multipacks and offers on their actual quantities
Use the total price you would pay and the total quantity you would receive, rather than the number of packs alone.
For a multipack, multiply the number of packs by the quantity in each one. Six 25g bags contain 150g altogether. Compare that total weight with the alternative rather than assuming six bags represent more product.
For an offer covering several packs, divide the total amount payable by their combined quantity. If you are choosing different products, check that a combined calculation remains meaningful: mixed sizes and product types can obscure the comparison.
Check any conditions, including the number you must buy and whether you qualify for the displayed price. If it is unclear which price the unit figure reflects, calculate from the amount you would actually pay or ask for clarification.
- Check the size of each individual pack.
- Confirm the quantity required for the offer.
- Compare with the single-pack option using matching units.
- Reject the extra quantity if it has no practical use for you.
Check what the stated quantity includes
Base your decision on the amount you will actually use, not just the largest number printed on the packaging.
Tinned products may show both net weight and drained weight. If you discard the liquid, drained weight can be the more relevant quantity for your own comparison. Check which weight the displayed unit price uses before comparing it with another product.
Likewise, a price per item can conceal differences in size. Two packs containing the same number of wraps are not necessarily equivalent if the wraps differ substantially in weight or dimensions.
Usable quantity also depends on your habits. A larger pack that leaves an unwanted remainder may offer less practical value than a smaller pack you will finish.
Know when unit price is not enough
Treat unit price as a comparison tool, not a complete judgement of value.
Unit price is less helpful when products need different amounts to do the same job. Concentrated squash and ready-to-drink squash, for instance, are not directly comparable by bottle volume alone. You would need to consider the preparation instructions and resulting drink quantity.
Similarly, whole vegetables and prepared vegetables may differ in preparation work and usable portions. Unit price can inform that choice, but it cannot put a value on convenience for you.
- Does the product meet the same need?
- Is the comparison based on an equivalent usable quantity?
- Will you use the extra amount?
- Does the total pack price fit what you want to spend?
Start with one useful comparison
On your next supermarket visit, try comparing two sizes of one product you buy regularly. Match the units, check the pack details and weigh the lower unit price against the amount you actually need.
You do not need to calculate every item in your trolley. Use unit pricing where the choice is unclear, then choose the suitable pack whose total price and quantity work for you.