Inflation Calculator

An inflation calculator shows how changes in a price index or an assumed inflation rate affect money over time. It can translate a past amount into an equivalent current amount, estimate a future price, compare wage growth with inflation, measure a personal spending basket and convert nominal savings into today’s purchasing power.

The result is a mathematical estimate, not an official inflation release, investment forecast or legal indexation decision. Reliable calculations begin with a consistent price-index series and clearly stated assumptions. The calculator deliberately does not contain a live inflation rate, because official data change every month and statistical agencies revise weights, classifications and base periods.

What consumer price indices measure

A consumer price index measures the average movement in prices for a defined basket of goods and services purchased by households. Items receive weights according to their importance in household spending, so a category with a larger expenditure share influences the headline index more strongly.

The index describes an average basket. It does not reproduce every household’s experience. A renter, homeowner, student, commuter and retired household can face different price changes because their spending patterns differ. This is why the calculator includes both an official-index mode and a personal-basket mode.

Official agencies maintain several measures. In the United Kingdom, the Office for National Statistics describes CPIH as its most comprehensive consumer-price measure because it includes owner occupiers’ housing costs. In the European Union, HICP provides a comparable measure across countries, while national indices remain important for domestic analysis and indexation.

Historical CPI index comparison

The most direct historical calculation uses two index values from the same series:

Equivalent target-period amount = base amount × target index ÷ base index.

The index ratio is 1.215, so cumulative inflation is 21.5%. The purchasing power of an unchanged nominal amount moves in the opposite direction:

Purchasing power of the same nominal amount = amount ÷ index ratio.

With the same example, 1,000 in the target period has the purchasing power of about 823.05 in base-period money. This does not mean every individual price increased by 21.5%; it reflects the weighted average represented by the selected index.

Annualise a multi-year change correctly

Cumulative inflation and annual inflation answer different questions. A five-year increase of 21.5% is not the same as 4.3% every year, because price changes compound.

Annualised inflation = (target index ÷ base index)^(12 ÷ months) − 1.

The formula produces the constant annual rate that would generate the same total index change over the stated period. Enter the actual number of months between observations. Using five years when the dates are only 54 months apart changes the result.

Base-year changes and rebasing

Statistical agencies periodically update classifications, weights and reference years. Rebasing changes the numerical scale of an index, not automatically the measured price movement. A ratio calculated within one continuous, officially linked series remains meaningful.

Problems arise when a user takes a base value from one version and a target value from another unrelated or unlinked version. Destatis, for example, explains that Germany’s consumer price index undergoes regular revisions and base-year changes. Statistics Sweden also updated the classification and reference year for its CPI from 2026. Use backcast or linked series supplied by the agency when methodology changes.

Choose the right index for the question

Index choice should match the intended comparison.

• National CPI: often used for domestic inflation analysis and indexation.
• HICP: designed for comparable inflation measurement across EU countries.
• CPIH: the ONS measure that adds owner occupiers’ housing costs to CPI coverage.
• Special household indices: may better represent particular income groups or household types.
• Personal basket: reflects only the categories and rates entered by the user.

Do not select an index merely because it gives a preferred result. Document why its scope fits the salary, contract, budget or research question.

Constant-rate future inflation projection

For planning, the calculator can apply a low, base and high assumed rate. These are scenarios rather than predictions.

Future cost = current amount × compounded inflation factor.

Annual compounding uses (1 + rate)^years. Monthly and daily compounding divide the nominal annual rate across more periods, while continuous compounding uses an exponential factor. For ordinary inflation scenarios, annual compounding is usually easiest to explain. Use another method only when the assumption or financial model requires it.

Negative rates are supported. A negative inflation rate represents deflation, but every periodic factor must stay above zero. A rate of −100% would reduce the price level to zero and breaks the compounding model.

Variable annual inflation rates

Historical and scenario rates often change from year to year. They must be multiplied, not added:

Cumulative factor = (1 + r1) × (1 + r2) × … × (1 + rn).

The calculator shows the cumulative factor after every period and converts the final factor into an equivalent annual rate. This mode is useful for historical annual rates, stress testing and budgets with different assumptions for each year.

Import a custom CPI or rate series

A user can paste period-and-value rows from an official source. For an index series, select the base and target periods after loading the data. For a rate series, the tool creates variable-rate rows.

Before importing, identify whether values are monthly indices, annual averages or year-on-year rates. Also confirm the decimal format and whether the series has been revised.

Calculate personal inflation

Official CPI uses population-level expenditure weights. The personal-basket mode replaces those weights with the user’s own annual spending.

Category weight = category spending ÷ total spending.

Each category grows by its own entered inflation rate. The calculator adds the future category costs and annualises the change in the total basket:

Personal inflation = (future basket ÷ current basket)^(1 ÷ years) − 1.

This remains an estimate. Broad categories contain products with different price movements, and households may change quantities or substitute items. ECB research confirms that household inflation can differ because expenditure shares differ.

Salary and purchasing power

A nominal pay rise does not guarantee a real pay rise. The calculator compares salary growth with the price-level factor.

Projected nominal salary = current salary × salary-growth factor.

Salary required to preserve purchasing power = current salary × inflation factor.

Salary in today’s money = projected salary ÷ inflation factor.

The mode uses gross salary and does not model taxes, pension contributions, benefits, promotion or contract rules.

Savings and exact real return

Inflation reduces the purchasing power of money even when the nominal balance increases. The calculator projects initial savings, monthly contributions, nominal return, annual fees and a simplified tax on investment gain.

The exact real-return formula is:

Real return = (1 + nominal return) ÷ (1 + inflation) − 1.

Subtracting inflation from the nominal return is only an approximation. The exact formula matters more when rates are large.

The tool distinguishes beginning- and end-of-month contributions. Beginning-of-month payments earn one extra return period. Fees and tax are simplified estimates, so actual outcomes can differ.

Reverse inflation calculation

Sometimes the start amount, end amount and time are known, but the implied rate is not.

Implied annual rate = (target amount ÷ base amount)^(1 ÷ years) − 1.

The calculator also derives an equivalent monthly rate and an approximate doubling or halving time. These are constant-rate equivalents, not descriptions of each intermediate period.

Currency and cross-country comparisons

Inflation calculation does not convert currencies. Comparing 1,000 pounds with 1,000 euros combines price change and exchange-rate effects. First keep the calculation in one currency, then perform a separate currency conversion for a specified date if required.

For cross-country inflation comparisons inside the EU, HICP offers harmonised definitions. National CPI series may remain more suitable for local wage clauses, rent rules or domestic purchasing-power analysis.

Practical data-checking routine

1. Choose the index that matches the purpose.
2. Use the same index series for both periods.
3. Record whether data are monthly, annual-average or year-on-year.
4. Confirm the base and target dates.
5. Enter the actual number of months between them.
6. Check for rebasing, revisions and methodological breaks.
7. Keep all monetary values in one currency.
8. Treat future rates as scenarios.
9. Compare personal-basket results with the official average rather than replacing it.
10. Save the source URL and index name with exported results.

Common mistakes

• adding annual inflation rates instead of compounding them;
• mixing CPI with HICP, CPIH or RPI;
• using values from unlinked base years;
• confusing an index level with an inflation percentage;
• using annual averages with monthly observations;
• assuming official CPI equals personal inflation;
• treating a scenario as a forecast;
• subtracting inflation from investment return when precision matters;
• comparing different currencies without exchange rates;
• using a gross salary result as a net-income forecast.

Frequently asked questions

Official and primary sources

• Office for National Statistics: consumer price indices technical guidance
• ONS: updating consumer-price weights
• Eurostat: Harmonised Index of Consumer Prices
• Eurostat: HICP methodology
• INSEE: indice des prix à la consommation
• Destatis: methodology for the German CPI
• Statistics Sweden: Consumer Price Index
• European Central Bank: household inflation differences

Sources reviewed: 23 June 2026.