Kitchen skills
How to compare grocery unit prices across pack sizes
Put different pack sizes on the same price-per-unit basis, then keep that calculation separate from the amount you need and the checkout total.
To compare differently sized packs, divide each pack price by its stated quantity, then express every result in the same unit. For grams, price ÷ grams × 100 gives a price per 100 g. The lowest result identifies the lowest unit price among those entries; it does not decide which pack suits your shopping list.
You need three pieces of information for each candidate: the product, the price you would actually pay and the quantity that price buys. Keep the checkout price visible beside the calculation. A larger pack can have a lower unit price while still requiring more money at the till.
Read the unit label before comparing the number
The GOV.UK — Competition and Markets Authority's consumer explainer on unit pricing, published in 2015, describes comparing prices using a common weight, volume or count. It explains that a larger pack is not necessarily cheaper per unit, and points readers towards unit prices displayed with product prices. We use it here for the comparison method, rather than as a statement of current pricing law.
Look for the unit attached to each displayed price. A figure per kilogram and a figure per 100 g cannot be compared as bare numbers. Nor can a price per item be read as a price per litre. Copy the labels first; a smaller-looking number on the shelf may simply use a smaller unit.
Choose candidates that answer the same shopping question. Different product forms or different stated quantities may need further information before a comparison is meaningful. If one label gives drained weight and another gives total weight, decide which quantity your question concerns and find matching information. Do not substitute a convenient number without noting the difference.
Put each pack on one basis
Our worked example uses three fictional packs of the same hypothetical product. Pack A costs £1.80 for 300 g, pack B costs £2.50 for 500 g, and pack C costs £3.30 for 600 g. These are invented figures, not current offers, observed supermarket prices or a promise of available savings.
For pack A, £1.80 ÷ 300 × 100 = £0.60 per 100 g. Pack B gives £2.50 ÷ 500 × 100 = £0.50 per 100 g. Pack C gives £3.30 ÷ 600 × 100 = £0.55 per 100 g. Keep the quantity and currency identical in the result column before ranking the rows.
On a narrow screen, scroll the table sideways. Keyboard users can focus it and use the arrow keys.
| Fictional pack | Checkout price | Stated quantity | Price per 100 g | Equivalent price per kg |
|---|---|---|---|---|
| A | £1.80 | 300 g | £0.60 | £6.00 |
| B | £2.50 | 500 g | £0.50 | £5.00 |
| C | £3.30 | 600 g | £0.55 | £5.50 |
Pack B has the lowest unit price in this table. Pack A has the lowest checkout price. Pack C is the largest pack, but its unit price is higher than B's. Those are three different observations. None tells us whether a particular product is suitable, whether its quality is comparable or how much you can afford to spend today.
Convert matching units consistently
There are 1,000 g in a kilogram, so a price per kilogram divided by ten gives the equivalent price per 100 g. In our table, £5.00 per kg becomes £0.50 per 100 g. Converting in the other direction means multiplying the price per 100 g by ten. Change the unit label along with the number.
For volume, use one common volume unit throughout. A comparison per 100 ml can be converted to one per litre by multiplying by ten. This does not provide a grams-to-millilitres conversion: mass and volume are different quantities. If a comparison requires information you do not have, keep it open instead of mixing the two labels.
For products sold by count, use price divided by the number of items only when an item is a meaningful basis for your comparison. The same item count does not establish the same individual size or specification. Note any difference you care about beside the price, so the unit calculation does not silently stand in for the whole choice.
Check what an offer actually buys
If you are considering an offer, record the total eligible price and the total quantity you would receive. Calculate from those inputs rather than from a promotional phrase. Check any membership, purchase-quantity or product conditions yourself before using the offer price; this worksheet cannot establish whether you qualify.
As a separate fictional example, two 300 g packs for a combined £3.00 contain 600 g. £3.00 ÷ 600 × 100 gives £0.50 per 100 g. That matches pack B's unit price above, while requiring a £3.00 checkout total rather than £2.50. The arithmetic describes only those imagined terms, with no real discount implied.
Bring quantity needed back into the decision
After comparing unit prices, look at your actual shopping list and cupboard record. How much of this product do you need to buy, and which pack sizes supply it? Record surplus quantity as surplus, rather than treating every gram in a larger pack as something you definitely need. Your intended use belongs beside the numbers.
If you want to compare complete shopping options, calculate their checkout totals separately. In our example, buying two packs A costs £3.60 for 600 g. One pack B costs £2.50 for 500 g. Those options deliver different quantities, so their totals alone cannot answer a question about a fixed amount without another decision about what is required.
Keep a comparison you can explain
A useful shopping note has columns for exact product, date, payable pack price, labelled quantity, common unit price and quantity needed. Leave a question mark where a condition or measurement is unclear. Keep any displayed unit price beside your own calculation if you want to check a transcription or rounding difference.
Our kitchen cupboard guide offers a starting point for recording what is already at home. Use that record with the comparison, then choose the purchase that answers your own list and spending constraint. The unit-price column supplies one precise piece of information; the amount to buy and the final checkout total still deserve their own lines.
