A shelf tag showing 0.36 per 100 g cannot be compared directly with another showing 3.90 per kilogram. The second number looks larger because the unit is larger. Before deciding which food costs less, put both prices on the same basis.
The same problem appears with multipacks, drained cans and offers that apply only when you buy several items. The calculations below use invented prices and package sizes, not current store prices or promised savings. Use the actual eligible price and quantity on the products in front of you.
Write the unit beside the number
The National Institute of Standards and Technology explains how unit pricing works: it expresses cost against a unit such as a gram, liter or ounce, allowing quantity comparisons. The unit is part of the information. A price per item and a price per kilogram answer different questions.
First confirm that the shelf label belongs to the package you are holding. Check the product, variant and size; similar-looking products can sit above the wrong label. If the label and package disagree or a promotion is unclear, ask store staff before relying on the calculation.
Compare like quantities. Weight can be converted to another weight unit, and volume to another volume unit. Do not treat 500 ml of a product as 500 g unless you have a valid basis for that specific product. For a straightforward grocery choice, comparing the same type of food using the units actually supplied is usually enough.
One conversion can resolve the comparison
Suppose two equivalent dry pasta products have the following hypothetical labels. Currency is kept the same throughout; these figures are simply arithmetic examples.
| Product | Displayed unit price | Price per kilogram |
|---|---|---|
| Pasta A | 0.36 per 100 g | 3.60 |
| Pasta B | 3.90 per kg | 3.90 |
There are ten portions of 100 g in a kilogram, so multiply 0.36 by ten. Pasta A has the lower unit price in this comparison. Alternatively, divide 3.90 by ten to express Pasta B as 0.39 per 100 g. Either direction works; changing the basis for only one label is what makes the comparison misleading.
If neither tag supplies a usable unit price, divide the price by the quantity. A hypothetical 750 g pack costing 2.85 is 2.85 ÷ 750 × 100, or 0.38 per 100 g. Keep extra decimal places until the last step when close prices matter, then round consistently.
Unpack a multipack on paper
For four identical 125 g tubs, the total net quantity is 500 g. At an invented price of 3.20, that is 0.64 per 100 g. A single 450 g tub costing 2.70 is 0.60 per 100 g. The multipack costs more by weight here, although individual sealed portions might still suit a particular use.
Check that each inner pack really has the stated quantity. “Six pack” is a count, not a weight. Comparing six small portions with six larger portions by price per item can obscure the difference. Likewise, compare eggs of the intended size and product type rather than treating any dozen as an identical purchase.
For canned food that you plan to drain, look for the drained weight when it is provided and make the basis consistent across the comparison. Do not compare one can’s total contents with another can’s drained contents. If the relevant quantity is unavailable, acknowledge that the exact usable-weight comparison remains unknown instead of inventing a yield.
Recalculate an offer using the purchase you qualify for
A multi-buy price applies only under its stated terms. If three identical packs cost 6.00 together and each contains 400 g, the qualifying purchase is 1,200 g at 0.50 per 100 g. That does not mean one pack costs 2.00 unless the offer explicitly permits it.
Check whether membership, a coupon, a particular flavor combination or a minimum quantity is required. Use the price you can actually obtain, not the largest printed saving. Do not include a loyalty discount in one side of the comparison while using an unavailable discount on the other.
The UK Competition and Markets Authority’s consumer unit-price guidance also illustrates why the biggest container is not necessarily cheaper by quantity. That arithmetic travels across markets; local pricing and consumer rules do not automatically do so. This guide is a comparison method, not legal advice about a particular shop.
Finish with the package you can use
A lower unit price is one input to a decision. Check dietary requirements, the food’s suitability, the cash you must pay today and whether you can store the purchase as instructed. Two products with different ingredients or preparation needs may not serve the same meal even after their prices are normalized.
Our guide to planning a tight grocery budget covers assigning packages to meals and avoiding unused food. Keep that separate from the label calculation: first establish the comparable price, then decide whether the quantity belongs in your basket. Save a short note for a repeated purchase only if it includes the product, quantity, price and date; an old number without its unit is not a reliable benchmark.
