The Math Behind It

The whole method is one division: price ÷ quantity. The trick is using the same unit for both sizes you compare.

Say a cereal box is $4.49 for 18 ounces, and a bigger box is $6.29 for 24 ounces. Divide each by its ounces:

• Small box: $4.49 ÷ 18 = $0.25 per ounce
• Big box: $6.29 ÷ 24 = $0.26 per ounce

The smaller box wins, by a hair. Two divisions, one clear answer.

A sale can flip the usual order. A small container at a markdown can easily beat a large one at full price, even though the large size is the better deal most weeks.

That is why the shelf price alone lies. Run the unit price whenever you compare brands, sizes, or anything tagged "sale" — the discount might be smaller than it looks, or bigger.