Starlight Tools

Inflation Calculator: Adjust Dollars by CPI from One Date to Another

Convert an amount between years or months using CPI observations. Choose the dataset and dates first; Fisher, forward-rate, and projection tools are available as advanced modes.

Modes:

CPI Inflation Calculator

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\(\displaystyle \text{Amount}_{end}=\text{Amount}_{start}\times\frac{\text{CPI}_{end}}{\text{CPI}_{start}}\).

Worked Examples

$100 from 2000 to 2020

Using CPI values of about 169.3 and 258.7, the formula is 100 × 258.7 / 169.3, giving about $152.80. That means prices rose by roughly 52.8%.

Deflating a current price

If an item costs $50 when CPI is 310 and you compare it with a past CPI of 250, the past-dollar equivalent is 50 × 250 / 310, or about $40.32.

Future Inflation Estimate

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Assumption-based estimate: \(\displaystyle \text{future amount}=A(1+r)^t\). This does not use historical CPI observations.

Expected Inflation (Fisher)

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Forward (ex-ante) via Fisher: exact \(\displaystyle \pi = \frac{1+i}{1+r}-1\); approximation \(\pi \approx i-r\).

Forward Inflation from Spots (m→n)

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\(\displaystyle (1+ \pi_{0,n})^n = (1+\pi_{0,m})^m \cdot (1+ f_{m,n})^{\,n-m}\Rightarrow f_{m,n}=\left(\frac{(1+\pi_{0,n})^n}{(1+\pi_{0,m})^m}\right)^{\!\frac{1}{n-m}}-1\).

Understanding CPI Inflation and Purchasing Power

Inflation affects the purchasing power of money: when the price index rises, the same nominal amount buys less. The common historical conversion task is to take an amount from one date, look up CPI at that date and another date, then multiply by the ratio of the two CPI values.

CPI adjustment is useful for salary comparisons, rent history, historical prices, budgets, and broad purchasing-power checks. It is not a personal cost-of-living measure: your own spending basket can differ from the national index.

The advanced tabs answer different questions. Projection mode estimates a hypothetical future or past amount from a fixed annual inflation assumption. Fisher and forward-rate modes are finance formulas for expected or implied inflation, not historical CPI conversion.

Methodology

  • Data sources: U.S. CPI-U uses FRED series CPIAUCSL. UK CPIH uses ONS inflation and price index data from the bundled page dataset.
  • Formula: converted amount = original amount × end CPI / start CPI. Cumulative inflation = end CPI / start CPI − 1.
  • Average annual rate: the calculator annualizes the CPI ratio across the elapsed time between selected observations.
  • Rounding: currency results are rounded to cents, percentages to four decimals, and CPI values to three decimals where available.
  • Update cadence: date selectors are generated from the CPI observations bundled with the site. The freshness note shows the latest loaded observation.
  • Privacy: calculations run in your browser. Amounts and selected dates are not uploaded.
  • Limitations: CPI is an average index for a broad basket of goods and services. It may differ from your personal cost of living, local prices, taxes, product quality changes, or official calculators that use different CPI series.

Inflation Calculator FAQ

Which CPI index does this calculator use?

The main calculator uses U.S. CPI-U All Items from FRED when United States is selected, and UK CPIH All Items from ONS when United Kingdom is selected.

Why can results differ from BLS or other calculators?

Different calculators can use different CPI series, seasonal adjustment choices, interpolation rules, release dates, and rounding. This page shows the CPI observations used in the result block.

Is the data seasonally adjusted?

The U.S. dataset is FRED CPIAUCSL, which is seasonally adjusted. BLS's own inflation calculator commonly uses CPI-U All Items, U.S. city average, not seasonally adjusted, so small differences are expected.

How are monthly dates handled?

For monthly datasets, the month and year selectors are generated from available observations. For annual datasets, the month selector is replaced with an annual setting.

What does deflation mean?

Deflation means the end CPI is lower than the start CPI. In that case, the converted amount and cumulative inflation percentage can be lower or negative.

Can I estimate future inflation?

Yes. Use the Projection tab for an assumption-based future or backward estimate using an annual inflation rate and number of years. Historical CPI mode uses observed data instead.

Can I use this for salary or purchasing-power comparisons?

Yes. Enter a salary, price, rent, or budget amount to compare broad purchasing power across dates. CPI is an average and may not match your personal spending pattern.

Do you use the exact or approximate Fisher formula?

The advanced Fisher tab shows both the exact formula, (1+i)/(1+r)-1, and the approximation, i-r.

5 Fun Facts about Inflation

Core isn’t “low” inflation

Core CPI just strips food and energy to see stickier trends; headline can run hotter or colder by several points in a single month.

Headline vs core

Rounding hides a lot

Monthly CPI is often rounded to one decimal. A 0.14% vs 0.24% move both read “0.2%” but annualize to different stories.

Decimal drama

Deflation boosts real growth

Negative inflation makes nominal growth look meeker than real growth—real GDP can rise faster than the money numbers suggest.

Price lift

Base effects are optical illusions

High or low prints a year ago can make today’s YoY inflation swing, even if monthly changes are calm. Always peek at month-on-month.

Compare carefully

Breakevens aren’t a “forecast”

Inflation swaps and TIPS breakevens embed risk premia and liquidity quirks—they’re a market price, not a crystal ball.

Market lens

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