Unit Price (Price per Unit Quantity)

The only arithmetic that makes two packages comparable: divide the price by the quantity and compare the results. Supermarkets in many jurisdictions are required to print it on the shelf tag precisely because shoppers otherwise cannot do it in their heads, and the required unit is often chosen to be awkward for exactly one of the two products.

The folk rule that the bigger box is cheaper per unit is true often enough to be dangerous. Studies of shelf pricing routinely find a quantity surcharge on the largest size in a fifth or more of product lines, usually where the large size is the one on promotion elsewhere or the one shoppers assume they need not check. The other trap is comparing across different measures: price per kilogram of a drained weight against price per kilogram of a net weight is not a comparison at all, and the same goes for concentrated versus ready-to-use.