How Inflation Changes Purchasing Power

Distinguish a scenario from historical evidence

This calculator applies a constant annual rate that you choose. It does not fetch a country’s current inflation rate, estimate a forecast or look up historical CPI. Its default 3% is an illustration. Choosing USD, CAD, GBP or AUD changes display formatting, not the assumed rate.
For a historical question, use a suitable official price index and the relevant dates. The Bank of England’s historical calculator uses UK CPI and explains that it gives an approximate guide to buying power. A national basket still need not match your personal spending or the price of a particular service.

Use the compound price factor

Write the annual percentage as a decimal: 3% becomes 0.03. For n years, the price factor is (1 + rate)^n. Future cost is today’s cost multiplied by that factor. Fixed-cash buying power in today’s money is the fixed nominal amount divided by it.
At 3% for 10 years, the factor is about 1.343916. A basket priced at 1,000 today would cost 1,343.92 under that scenario. A fixed cash amount of 1,000 would buy what about 744.09 buys today. The difference reflects two questions, not conflicting answers.

Run a planning comparison step by step

1. Identify an expense in today’s prices or a cash amount you expect to remain fixed.
2. Enter a rate as an explicit assumption and choose the time horizon.
3. Read the future-cost figure if you are estimating how a spending target might change.
4. Read the buying-power figure if you are examining a fixed nominal amount.
5. Repeat with other plausible assumptions rather than relying on one precise-looking number.
Keep all inputs in one currency. If you also model investment growth, compare nominal and inflation-adjusted values using the same dates and distinguish returns before and after any relevant charges or taxes.

Compare illustrative rates over ten years

These scenarios all start with 1,000 and use annual compounding for ten years. They are mathematical illustrations, not predictions or official rates for any country.

Understand cumulative growth and buying-power loss

Cumulative price change is (factor − 1) × 100%. At 3% over ten years it is about 34.3916%, not 30%. Each year applies the rate to the previous year’s price.
The reduction in fixed-cash buying power is (1 − 1/factor) × 100%, about 25.5906% in this example. A 34.3916% price increase does not imply a 34.3916% loss of purchasing power because the percentages use different bases. A simple check is a doubling of prices: the same cash buys half as much.

Check boundaries and avoid overinterpretation

Zero years or a zero rate leaves amounts unchanged. A negative rate models deflation, reducing future prices and increasing fixed-cash buying power. The calculator permits rates from −50% to 100% and horizons up to 100 years to keep inputs within a defined range. These bounds are not an economic forecast. Fractional years use the same compound formula.
Do not multiply a multi-year horizon by a percentage and assume that gives compound growth. Do not mistake slowing inflation for falling prices. Avoid rounding the factor before using it: calculate with the full value and round the final money amounts. Above all, treat a scenario as a way to compare assumptions rather than a promise about future costs.

Common questions

Sources

• Bank of England: inflation calculator and method