The only price that matters is per unit
A package price answers the wrong question. The comparable number is price divided by size: per ounce, per gram, per litre, per sheet. Once both packs are in those terms, the comparison is exact regardless of how the packaging changed.
The Bureau of Labor Statistics works the same way. Its published example: a tub of ice cream falls from 64 to 60 ounces while the price stays at $5.99, so the price per ounce goes from about $0.094 to $0.0998 and the CPI records a 6.7% price increase.
Counting works the same as weighing
Items sold by count shrink just as easily. BLS's second example is a toilet paper roll going from 220 sheets to 200: the economist adjusts the data to show a 10% increase in the price per sheet, even though the shelf price never moved.
Household paper products are the category where BLS has recorded the most size changes, up and down. Snacks, cereal and cleaning supplies follow. Products sold by weight or volume at the counter — gasoline, steak — cannot be downsized this way, because you are already buying the unit.
Worked example
220 → 200 sheets at the same price
220 ÷ 200 − 1 = 10% per sheet
When price and size move together
The two effects multiply rather than add. A 10-ounce bag of chips at $4.29 that becomes 9 ounces at $4.49 shows 4.7% more on the tag, but 42.9¢ an ounce becomes 49.9¢ an ounce: a real increase of 16.3%.
The relationship is exact, and it is worth memorising in one line: one plus the real change equals one plus the shelf change divided by one plus the size change. A 10% smaller pack on its own is an 11.1% price rise, because the same money now buys 0.9 of what it did.
How small a cut hides a rise
Run the arithmetic backwards and you can see why particular sizes appear. To hide a 6.7% increase at an unchanged price, a 16-ounce pack has to fall to 14.995 ounces — in other words, to the 15 ounces a new label would show.
That is the practical reason downsizing is popular: research cited by BLS finds shoppers notice price changes more than size changes. A one-ounce difference on a label is easy to miss; a 40-cent difference on a tag is not.
It is already in the inflation rate
Because BLS adjusts for size, downsizing pushes the CPI up exactly as a higher tag would. Food and beverage prices rose 2.6% in the year to August 2026 with any downsizing counted inside that figure.
BLS has also published research indexes that deliberately ignore size changes, so the gap between those and the CPI measures how much downsizing contributed. Shrinkflation is not missing from official inflation; it is one of its inputs.
Using it in a shop
The same two divisions answer the everyday question of which pack is better value: enter the two packs as before and after, and a positive result means the second costs more per unit.
Shelf labels often do this for you, but only when both use the same unit. When one shows price per 100g and the other price per item, convert first and compare like with like.
Common mistakes
Comparing shelf prices between pack sizes.Only price per unit compares; the tag alone tells you nothing about value.Treating a smaller pack at the same price as no change.It is a price rise, and the CPI records it as one.Adding the price change and the size change.They multiply: 4.7% dearer and 10% smaller is 16.3%, not 14.7%.Comparing different units.Convert fluid ounces to millilitres, or per-item to per-100g, before comparing.