How to Calculate Shrinkflation from Package Sizes
An unchanged price can still buy you less.
When a package gets smaller while its price stays the same, the price for a fixed quantity increases. This is commonly called shrinkflation. NIST explains how unit pricing helps shoppers compare products and spot the effect of smaller packages.[1] A price change and a size change can also happen together.
Use the old package as A and the new one as B
You need four observations: the old price and quantity, and the new price and quantity. A receipt can establish the price; the package label can establish the contents. Match the product variety and purchasing conditions. Comparing an old promotional offer with a current regular price also measures the loss of that promotion.
This is a price comparison
The result describes the change in cost per quantity. It does not decide whether a package is misleading under a particular law, nor does it evaluate ingredient or quality changes. Keep those questions separate.
Worked example: same price, smaller package
Take an invented example: the old package contains 16 ounces and costs $3.00. The new package contains 12 ounces and still costs $3.00. These figures are chosen to explain the calculation; they are not a claim about a named brand.
The old unit price is $3.00 per pound. The new one is $4.00. The quantity change is -25 percent, while the unit-price change is +33.33 percent. Those percentages use different starting points and should not be treated as interchangeable.
Compare your packages
Package A per lb (pound)
$3.00
Package B per lb (pound)
$4.00
Change from A to B
+33.33 %
At checkout: $3.00 for A or $3.00 for B. A lower unit price does not automatically mean a smaller bill.
| Package | Quantity | Price |
|---|---|---|
| A | 16 oz (weight) | $3.00 |
| B | 12 oz (weight) | $3.00 |
Assumptions behind this calculation
- The comparison uses the same kind of quantity without intermediate rounding. Unit prices display up to four decimal places. It assumes equivalent products and full use. The package price remains what you pay at checkout.
The full calculator with an example, the method and sources: Unit Price Calculator: Compare Package Sizes
The formula uses unit prices, not just package prices
Calculate each unit price first. Divide the new unit price by the old unit price, subtract one, then multiply by one hundred for the percentage change. This method works when the price also changes. A reduction in contents alone does not tell you the full price increase if the new package costs a different amount.
| Observation | What to check |
|---|---|
| Same price, smaller quantity | The unit price rises for the same product |
| Both price and quantity change | Calculate both unit prices |
| A larger-looking box | Read the actual labeled contents |
| Different formula or serving size | Check performance or quality separately |
Keep a comparison you can verify later
Record the product name, variety, price, quantity, store and date. Do not rely on the shape of the box or on a memory of its old price. For items sold by count, make sure the individual items are still comparable. A roll count does not describe the same quantity if the number of sheets per roll changed.
Compare alternatives before stocking up
A measured increase is a reason to compare, not an instruction to buy more. Use unit prices to check alternative products and the bulk-buying guide to account for unused contents. The better purchase still has to fit your grocery budget and actual needs.
What you can do about it
-
Keep the old figures
Record price and labeled contents together with a date.
-
Compare full unit prices
Use both price-quantity pairs, including any simultaneous price change.
-
Check alternatives
Compare actual need, product quality and cash required at checkout.
Frequently asked
Can I calculate shrinkflation if the price changes too?
Yes. Enter the price and quantity for each version. The calculation compares their unit prices.
Does this prove a package is illegal?
No. It measures price per quantity and makes no legal judgment about the packaging.
What if the original item was free?
A percentage increase from a zero starting price is undefined. The calculator shows the prices without inventing a percentage.
Sources
- Uniform Unit Pricing: Tools for Consumers to Fight Shrinkflation, National Institute of Standards and Technology, retrieved October 2, 2026.
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