Unit Price Comparison Calculator: Per kg, Litre and Item
Unit Price Comparison Calculator
Three offers, any mix of grams, kilograms, pounds, litres, gallons or plain counts, reduced to one comparable number each: price per kilogram, per litre or per item. The cheapest is flagged, and the mixed-unit trap, comparing a per-pound price to a per-kilogram one by eye, goes away.
Conversions used: 1 lb is 0.45359 kg, 1 oz is 28.35 g, 1 US gallon is 3.785 L, 1 imperial gallon is 4.546 L, 1 US fluid ounce is 29.57 ml. Mass offers compare per kilogram, volume offers per litre, counted items per unit; the tool only declares a winner among offers measured in the same dimension.
How to use this tool
Enter up to three offers, each with its quantity in whatever unit the shelf uses, grams, kilograms, pounds, ounces, millilitres, litres, US or imperial gallons, or a plain count, and its price. Every offer is reduced to a price per kilogram, per litre or per item, and the cheapest within each dimension is flagged.
Why the shelf makes this hard on purpose
The same product routinely appears in metric on one label and imperial on the next, 750 grams here, a pound and a half there, and the per-unit stickers, where they exist at all, often use different bases on adjacent shelves. Comparing by eye across a pound-to-kilogram boundary requires dividing by 0.4536 in your head, which nobody does at the shelf, and the pricing counts on it.
The tool normalizes everything: mass to kilograms, volume to litres, counts to each. It also refuses to compare across dimensions, because a price per litre and a price per kilogram are only comparable through a density that depends on the product, so a winner is declared within each dimension present and never between them.
A worked check you can repeat on paper
The default offers: 750 grams at 3.00 is 3.00 over 0.75, which is 4.00 per kilogram. A kilogram and a half at 5.25 is 3.50 per kilogram. One pound at 2.00 is 2.00 over 0.45359, which is 4.41 per kilogram, the most expensive of the three despite the smallest sticker. Offer B wins, and the pound offer that looks cheapest by sticker price is dearest by unit price, which is the entire lesson of the tool in one row.
Mistakes that catch people out
Judging by sticker price. The smallest number on the shelf is usually the smallest package, not the best value. Fix: per-unit price is the only comparable number.
Missing the pound-kilogram boundary. A per-pound price is 2.2 times smaller-looking than the same per-kilogram price. Fix: the tool converts; the 0.4536 lives in the code, not your head.
US versus imperial gallons. They differ by 20 percent, 3.785 versus 4.546 litres, and both appear in North America. Fix: they are separate units in the selector.
Ignoring usable quantity. The bigger pack wins per unit and loses if a third of it spoils. Fix: enter the quantity you will actually use.
How this was checked
The conversion set was verified against exact definitions: the pound at 0.45359237 kilograms, the ounce at 28.3495 grams, the US gallon at 3.785411784 litres and the imperial at 4.54609, with the worked offers above confirmed to the stated per-kilogram prices and the count case, six items at 3.00, returning 0.50 each.
Related Calculators and Reference
See also the Volume and Capacity Converter for converting the package volumes before comparing their unit prices.
Converting the quantities themselves between units is the Mass and Weight Converter and the Volume Unit Converter. What the winning offer costs after a discount and tax is the Discount and Sales Tax Calculator.