Divide the pack price by the amount you have a written plan for, not by the weight printed on the pack. The shelf shows price per kilogram of pack; the number that decides the purchase is price per kilogram of food you will actually eat, and the gap between them is real money: edible food thrown away in UK homes in 2022 came to 65 kilograms per person worth £260 at 2022 prices, which is £260 ÷ 65 kg = £4.00 a kilogram, in WRAP's UK food waste key facts. The condition that flips the answer is a credible plan for the remainder: with one, the large pack is the cheaper pack again.
Divide by the planned amount
Start from the quantity the meals need, written down before the shop. A pack is a manufacturing decision, not a portion instruction, and treating it as one moves the arithmetic after the purchase instead of before it.
Two people, two jobs. One writes the amount each planned meal needs and totals it; the other reads the shelf label and does the division in front of the shelf. Both numbers are said aloud before anything goes in the trolley.
Three packs against one planned amount
Two people plan meals needing 400 grams of an ingredient. The pack sizes and prices below are an example rather than a survey of shop prices, since no public source lists one shop's shelf prices; the per-kilogram value of the unused part is the UK 2022 figure of £260 for 65 kilograms per person in WRAP's key facts.
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| Pack | Price | Shelf price per kg | Cost per kg of the 400 g planned | Left over |
|---|---|---|---|---|
| 400 g | £2.40 | £2.40 ÷ 0.4 = £6.00 | £2.40 ÷ 0.4 = £6.00 | none |
| 750 g | £3.75 | £3.75 ÷ 0.75 = £5.00 | £3.75 ÷ 0.4 = £9.38 | 350 g |
| 1 kg | £4.50 | £4.50 ÷ 1.0 = £4.50 | £4.50 ÷ 0.4 = £11.25 | 600 g |
The shelf ranks them large, medium, small. The planned-use column ranks them in exactly the opposite order, and the checkout agrees with the shelf: the kilogram pack costs £4.50 − £2.40 = £2.10 more on the day. Both rankings are correct answers to different questions.
What the left-over column is worth
If the 350 grams from the 750 gram pack is never eaten, it is 0.35 kg × £4.00 = £1.40 of food. The 600 grams from the kilogram pack is 0.6 kg × £4.00 = £2.40. Add either to the purchase and the large packs get worse still.
If there is a written plan for the whole 750 grams — a named meal, on a named day, with the people who will eat it — the large pack returns to £3.75 ÷ 0.75 = £5.00 a kilogram of food used, and it is the better buy by £1.00 a kilogram. Write that plan down first. Label each remainder with one of three words: agreed, uncertain, or none, and resist promoting uncertain to agreed because the offer looks good.
The label date sets the deadline for the plan
A plan for a remainder is only as good as the time it has. The European Commission treats "use by" as an indicator of safety and "best before" as an indicator of quality, and estimates from a 2018 study that up to 10% of food waste generated annually in the EU is linked to date marking, with the same market study finding the date mark poorly legible on 11% of products sampled (date marking and food waste prevention). Follow the storage and opening instructions on the pack and local food-safety guidance; a date is not extended to make arithmetic work.
Past the date on the label accounted for 758,000 tonnes, 17% of the 4.4 million tonnes of edible food UK households binned in 2022, worth £3.15 billion (key facts). That is the column the large pack tends to fill.
Constraints the shelf label cannot show
Ask each other what actually limits the choice today: what can be carried home, what fits in the cupboard or freezer, how much cash is available this week, and when the next shop happens. Someone buying for two households has a different use plan from someone feeding one person, and neither situation belongs to a generation.
The rule that follows is short. Buy the pack that covers the agreed amount within those constraints; move up a size only when the remainder already has a meal, a day and an eater. Loose quantities solve the problem where they are sold, which is not everywhere.
Packaging stays outside this calculation. Comparing the overall environmental impact of two pack sizes needs evidence on the packaging, production, transport and disposal of the actual products, and the EU-level context for the food itself is 58.2 million tonnes of food waste in 2023, around 130 kilograms per person, per Eurostat's release of 16 October 2025.
On the next trip, compare the predicted use with what happened, and feed the correction into the shared list described in one shopping list for changing plans or into the plan comparison in compare two meal plans without judging the cook.
Sources
- UK Food Waste & Food Surplus – Key Facts, July 2025Accessed September 17, 2026
- Date marking and food waste preventionAccessed September 17, 2026
- 130 kg of food wasted per person annually in the EUAccessed September 17, 2026