How unit prices are compared
The shelf price tells you what one package costs. The unit price tells you what the product inside costs, which is the number that matters when sizes differ. To get it, divide the price by the quantity, with the quantity expressed in one agreed unit for every package you compare.
The calculator does this for up to three items (A, B and an optional C). For each one you enter the Price, the Size and the Unit printed on the pack: g, kg, oz or lb for things sold by weight; mL, L, US fl oz or US gal for liquids; or items for things sold by count. Then:
- Every size is converted into a common unit: kilograms for weights, liters for volumes, single items for counts.
- Each price is divided by its converted size, giving a price per kg, per L or per item. Weights are also shown per lb and volumes per US gallon, for readers used to those units.
- The lowest unit price is marked as the cheapest, and every other item shows how much dearer it is in percent.
You can mix grams with pounds or milliliters with US fluid ounces, because the conversion uses exact factors (one pound is 0.45359237 kg and one US gallon is 3.785411784 L). You cannot compare a weight with a volume: 1 kg of yogurt and 1 L of yogurt are different amounts, and turning one into the other needs the product’s density, which the tool does not guess.
Formula
The percent is measured against the cheapest item, so “4.91% dearer” means that item’s product costs 4.91% more per kilogram, liter or item than the best one.
Worked examples
- 500 g is 0.5 kg: 3.49 ÷ 0.5 = 6.98 per kg.
- 1.2 kg: 7.99 ÷ 1.2 = 6.658 per kg.
- 750 g is 0.75 kg: 4.99 ÷ 0.75 = 6.653 per kg.
- The biggest bag is not the best buy here, and the two larger ones are practically tied: the difference is less than a tenth of a percent.
- A pound is 16 oz, so 12 oz is 0.75 lb: 3.29 ÷ 0.75 = 4.387 per lb.
- The 1 lb pack: 4.19 ÷ 1 = 4.19 per lb.
- Per kilogram the same comparison reads 9.671 against 9.237; the ranking and the percent do not change with the unit.
- Treat the offer as one item: price 5.00, size 1,200 g (three 400 g cans). 5.00 ÷ 1.2 = 4.167 per kg.
- One can: 1.89 ÷ 0.4 = 4.725 per kg.
- The offer only saves money if you will use all three cans before they spoil or expire.
Why bigger is not always cheaper
Many shoppers assume the largest pack has the lowest unit price. Often it does, but not always: a promotion on a small size, a store brand in a different size, or simply a pricing decision can reverse the order. In one comparison, 2 L of a drink at 3.20 works out at 1.60 per L, while a 500 mL bottle at 0.75 costs 1.50 per L, so the large bottle is 6.67% dearer. Only the unit price shows this.
Why shelf labels use different units
Retail unit prices are not shown in one universal unit. In Great Britain, the Price Marking Order 2004 defines the unit price as the price per item for goods sold by number, and otherwise per kilogram, liter, meter, square meter or cubic meter as appropriate. Until 6 April 2026 a schedule to the order let listed products, such as spices, coffee and biscuits, be priced per 10 g or 100 g; an amending order removed that schedule from that date. In the United States, some states have unit pricing rules and elsewhere it is voluntary. NIST’s best-practice guide recommends using the same unit for every product in a category, choosing the one most familiar to shoppers, and its examples include cereal priced per 100 g.
Small quantities explain the per 100 g habit. A spice at 1.95 per 100 g is 19.50 per kg, a figure that looks alarming even though you will never buy a kilogram. Whatever unit a label uses, compare like with like: per 100 g against per 100 g, per kg against per kg. If two labels use different units, put both packages into this calculator instead.
Mistakes that skew a price comparison
- Comparing a weight label with a volume label. Some products, such as detergents or ice cream, are sold by weight in one brand and by volume in another. The calculator refuses the mix rather than inventing a density.
- Reading the drained weight as the net weight, or the other way round. For canned goods packed in liquid, decide which one you care about and use it for every item.
- Forgetting the count inside the pack. A pack of 6 rolls at 4.80 is 0.80 per roll; a pack of 9 at 6.75 is 0.75 per roll. Enter the count with the items unit, not 1.
- Counting a multi-buy you will not finish. A lower unit price on food that ends up thrown away is not a saving.
- Mixing up US and imperial fluid ounces. The tool uses the US fluid ounce (about 29.57 mL). A UK label in imperial fluid ounces (about 28.41 mL) should be converted first, or entered in mL.
The calculator compares prices only. It cannot judge quality, how much you will use or how long it keeps, which can matter more than a few percent.
Sources
- Unit Pricing Guide: A Best Practice Approach to Unit Pricing (NIST SP 1181), National Institute of Standards and Technology.
- The Price Marking Order 2004, article 1 (definition of unit price), legislation.gov.uk (The National Archives).
- Guide for the Use of the International System of Units (SI), NIST SP 811, Appendix B, National Institute of Standards and Technology.