Inflation Calculator — US and UK Purchasing Power Over Time

Supported inflation series

The calculator supports two headline consumer price indices compiled by official statistical agencies:

• US CPI (BLS): all-items Consumer Price Index for urban consumers, published by the U.S. Bureau of Labor Statistics.
• UK CPI (ONS): all-items Consumer Prices Index, published by the UK Office for National Statistics.

Data are loaded as annual CPI index levels by calendar year. The earliest and latest years available depend on the country series.

What is inflation?

Inflation is a general rise in prices over time and a fall in the purchasing power of money. When inflation is positive, the same amount of cash buys fewer goods and services in the future than it does today. CPI tracks this change using a representative “basket” of household spending.

The opposite is deflation, a general fall in prices. In deflation, a fixed amount of money buys more over time. Very high inflation rates over short periods are often called hyperinflation.

How the calculation works

We map both dates to calendar years and look up the CPI index for each year. If the start year has index CPI_start and the comparison year has index CPI_end, then an amount A in start-year prices corresponds to:

A × CPIend / CPIstart

For the yearly table, we step year by year between the two dates. Each year’s CPI change is applied to the amount: inflation increases the nominal figure; deflation decreases it.

Worked example

Suppose you want to know what $23 in 1975 would be in 1985 prices. If the CPI index in 1975 is 18 and the index in 1985 is 45, then:

$23 × 45 / 18 ≈ $57.50

This means the same basket of goods and services that cost $23 in 1975 would cost roughly $57.50 in 1985, based on the CPI index path.

Average annual inflation rate

The calculator reports an average annual rate of inflation between your two dates. If V_start is the starting value, V_end is the final value and n is the number of years between them, the average annual rate r satisfies:

Vend = Vstart × (1 + r)n

Rearranging gives:

1 + r = (Vend / Vstart)1/n

This geometric average smooths out year-to-year volatility into a single per-year figure that is easier to compare across periods.

Forward and backward calculations

• Forward: “What would $100 in 1950 be in 2010 prices?”
• Backward: “What would $100 in 2010 correspond to in 1950 prices?”

Both directions use the same CPI ratio, so the math stays consistent even when inflation is low, high or negative.

Date range and coverage

The earliest and latest dates you can select are limited by the underlying CPI series. If you choose a date outside the supported range, the calculator will not extrapolate.

Reading the results

• The summary shows starting amount, final value, total and percentage change, plus an average annual rate.
• The chart plots the inflation-adjusted value over time and the annual inflation rate (0% as the baseline).
• The table lists each year, its inflation rate, the change on your amount and the updated value.

Important assumptions & limitations

• Average basket: CPI reflects an average household basket; your personal spending may differ.
• Method changes: CPI methods and weights are revised over time, especially across long spans.
• No wages or investments: the tool only adjusts a fixed amount by CPI and does not model wage growth or returns.
• No extrapolation: results are not shown beyond the last available CPI year.

Next steps

• Project investments: use the ROI Calculator.
• DCA scenarios: explore recurring investing with the DCA Calculator.
• Backtests: compare inflation with market returns using the Crypto Calculator or S&P 500 backtest.