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Macroeconomics

CPI Inflation Calculator

Compare two observations of one CPI series, distinguishing month-on-month, year-on-year and custom-period price changes.

User inputs or hypothetical examples · Consistent units and periods · Local calculation

Inputs

YoY compares the same month a year earlier.

Previous month

1900–2200

Current month

1900–2200
More data settings

A basic comparison assumes one consistent index series. For published statistics, check the country, series, base and seasonal adjustment here.

Results

Start with your numbers

Review or adjust your inputs, then calculate to see your results and interpretation.

Understand this calculation

The index describes a price level relative to its reference, while the change rate describes movement over your chosen period. Index points and percentages are different measures.

See calculation method and assumptions

Formula

CPI Inflation

Two-period price change

Basket with fixed base expenditure weights

CPI measures changes in prices paid by consumers for goods and services. Its two-period inflation rate is the current index divided by the previous index minus one; our teaching basket weights price relatives using fixed base-period expenditure shares.

Variables, rates and periods

CPI0
Previous index in the same series (points)
CPI1
Current index in the same series (points)
π
Two-period decimal change
wj
Item j's base expenditure weight
pj,0
Item j's base price
pj,1
Item j's current price
Units
Indices use points; π is decimal, so 0.03 displays as 3%. MoM is one month, YoY twelve; custom changes are not automatically annualized.

Slower inflation still means rising prices

  • From 100 to 103 to 105, the second increase is smaller: disinflation.
  • Prices have not returned to 100.
  • Deflation requires a falling index, such as 105 back to 103.
  • Compare equal period lengths and consistent seasonal-adjustment conventions;
  • a one-month change is not an annual inflation rate.

CPI, core CPI and the GDP deflator

  • CPI follows household purchases and can include imported consumer goods.
  • The GDP deflator follows domestically produced final output, with different coverage and weights.
  • A common core CPI excludes food and energy to examine price movements beyond some volatile components, but check each publisher's definition.
  • It does not imply food and energy are unimportant or represent every person's living costs.

What the fixed basket can and cannot show

  • The interaction normalizes base spending to 100 and averages price relatives with base weights.
  • Changing weights creates a different hypothetical household, not a revision to official CPI history.
  • Official methods include sampling, quality adjustment, substitution and aggregation procedures;
  • these three categories cannot reconstruct Taiwan's or the US official CPI.

Compare two observations of one CPI series, distinguishing month-on-month, year-on-year and custom-period price changes.

Worked example

  1. Set two observations of one series to 100 and 103.
  2. Check dates: MoM requires one month and YoY twelve months.
  3. The increase is 3 index points and 3%, not 103% inflation.

Frequently asked questions

Why do my expenses differ from CPI?

Spending weights, location, products and quality can differ; official CPI is an average for a defined population.

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