Cereal 18→15.5 oz
- 4.99 → 5.29
- weekly
- CPI 3%
23.1%
Hidden yearly cost ≈ 59.97 at weekly buys.
How much are you really losing on a smaller pack?
This tool is a shrinkflation detector: it compares old and new pack size and price and shows the real hike. The calculator shows how much a product really rose when the pack got lighter or the price per ounce climbed. At 18 oz for 4.99 and 15.5 oz for 5.29 the true rate is 23.1%, and weekly buys hide about 59.97 per year. The result also compares that rise with a simple CPI benchmark.
The calculator shows how much a product really rose when the pack got lighter or the price per gram climbed. At 500 g for 5.00 and 450 g for 5.00 the true rate is 11.1%, and weekly buys hide about 28.89 per year. The result also compares that rise with a simple CPI benchmark.
Result
Enter quantities and prices.
This is unit price and household hidden cost, not official CPI. Related: inflation calculator and cooking vs takeout.
How it works
P/Q5.29/15.5
P/Q5/450
P/Q4.99/18
P/Q5/500
Rate23.1%
Rate11.1%
The model compares unit prices: price ÷ quantity after converting to a shared base for the unit you pick in the form (and in the header defaults). True shrinkflation rate = (new unit price − old) ÷ old × 100. At 4.99 / 18 oz and 5.29 / 15.5 oz the rate is 23.1%.
The CPI gap is that rate minus the benchmark you enter (default 3%), in percentage points. Annual hidden cost assumes you still buy the old quantity at the new unit price, times purchases per year.
It is unit-price arithmetic — the same idea statistical offices use when they adjust basket items for quality. The calculator does not replace official CPI.
Unit price
Shrinkflation is when the sticker price stays put while the pack holds less. The calculator ignores the label alone — it divides price by quantity on both sides and shows the percentage gap. If the shelf price also rose, the unit rate captures both effects.
Annual hidden cost answers: how much extra you pay if you still buy the old amount of product at the new unit price. The CPI benchmark is only a percentage-point comparison with a number you enter.
Units and basket
The package unit follows the header (US: oz; metric: g, kg, ml, L). All choices convert to a shared base before comparison. The optional basket sums annual hidden cost across several products at the same purchase frequency; each row has its own rate while the hero result stays on product one.
How less fill changes the rate
The table holds an old 18 oz pack at 4.99 and a new shelf price of 4.99. Only the new quantity changes.
The table holds an old 500 g pack at 5.00 and a new shelf price of 5.00. Only the new quantity changes.
| New qty | Rate | Gap vs 3% |
|---|---|---|
| 17 oz | 5.9% | 2.9 pp |
| 15.5 oz | 16.1% | 13.1 pp |
| 14 oz | 28.6% | 25.6 pp |
| New qty | Rate | Gap vs 3% |
|---|---|---|
| 480 g | 4.2% | 1.2 pp |
| 450 g | 11.1% | 8.1 pp |
| 400 g | 25.0% | 22.0 pp |
Shelf math 2026
Most shoppers notice a lighter box before they search the word “shrinkflation”. The calculator turns that into a unit price: price ÷ quantity. Cereal at $4.99 / 18 oz versus $5.29 / 15.5 oz is a 23.1% true unit rise — far above a 3% CPI benchmark in the comparison field.
Weekly hidden cost annualizes how much more you pay to get the old quantity at the new unit price. On the cereal example that is about $59.97 per year for one SKU.
Formula
Rate = (P₂/Q₂ − P₁/Q₁) ÷ (P₁/Q₁) × 100. Hidden cost per equivalent purchase = Q₁ × (P₂/Q₂) − P₁. Annual × purchase count. CPI gap = rate − benchmark.
What the numbers mean
- unit price
- Price ÷ quantity on a shared base.
- CPI gap
- Rate minus your benchmark, in percentage points.
- hidden cost
- Extra to keep buying the old quantity at the new unit price, annualised.
How to use
- Enter old and new quantity and price.
- Choose purchase frequency and CPI benchmark.
- Optionally enable the basket and add more items.
- Copy the summary or the link in the address bar.
Examples
Ice cream 16→14 oz
- 5.49 → 5.79
- weekly
- CPI 3%
20.5%
Hidden yearly cost ≈ 58.61.
Drink 67.6→60.8 oz
- 2.29 → 2.49
- monthly
- CPI 3%
20.8%
Hidden yearly cost ≈ 5.72.
Soda 2000→1800 ml
- 2.29 → 2.49
- monthly
- CPI 3%
20.8%
Hidden yearly cost ≈ 5.72.
Frequently asked questions
What does a 23.1% true shrinkflation rate mean?
It is the rise in price per package unit. At 18 oz for 4.99 and 15.5 oz for 5.29, the unit price rises 23.1%. With metric units, 500 g for 5.00 and 450 g for 5.00 is an 11.1% rise when the shelf price is unchanged.
What is the gap versus CPI?
It is the percentage-point difference between your unit-price rise and the CPI benchmark you enter (default 3%). It is not an official statistics-office index.
How is annual hidden cost calculated?
The calculator assumes you still buy the old quantity of product at the new unit price, then multiplies the gap by purchases per year. Weekly buys in the 500→450 example come to about 59.97.
Can I compare several products?
Yes. Turn on the shrinkflation grocery basket and add a second and third item. Annual hidden cost sums every row at the same purchase frequency.
Which units are supported?
oz, g, kg, ml and L. The header default is US (oz) on English pages and metric on Polish pages. The calculator converts your choice to a shared base before comparing unit prices.
Does it work when the sticker price also rose?
Yes. Both sides use price per unit. A smaller and more expensive pack produces a higher rate than weight loss alone.
Does this replace official inflation?
No. It is a tool for one product or a small basket. CPI covers a broad basket of goods and services.
Does the result update live?
Yes. After you enter quantities and prices, the result refreshes as you go. The page URL stores the same numbers for sharing.
Sources
- ONS: inflation and price indices
- BLS: Consumer Price Index
- GUS: wskaźniki cen
- Wikipedia: shrinkflation
Page updated October 15, 2026.