UTILITY / ECO-05
Inflation Calculator
Compare purchasing power using starting and ending consumer price index values.
Comparing purchasing power with price indexes
A price index summarizes changes in prices for a defined basket. This calculator compares two index values you enter; it does not select calendar years, download current data, or verify that the inputs come from the same series.
Adjusted value = starting amount × (ending index ÷ starting index)
Nominal increase = adjusted value − starting amount
Cumulative inflation = (ending index ÷ starting index − 1) × 100
If the index rises, a larger amount is needed in the later period to match the earlier amount’s purchasing power for that basket. The adjusted amount is not investment earnings or a personalized cost-of-living forecast.
Worked price-index examples
An illustrative rise from 100 to 120
With a $1,000 starting amount, $1,000 × 120 ÷ 100 = $1,200. The nominal increase is $200 and cumulative inflation is 20%. These are hypothetical index values, not a historical inflation claim.
Reversing the comparison
For $1,200 with a starting index of 120 and an ending index of 100, the adjusted value is $1,000. The change is −$200 and −16.67%, not −20%. The starting index determines the denominator.
No index change
Starting and ending indexes of 100 produce an adjusted value equal to the starting amount and 0% cumulative inflation. A falling ending index produces a negative nominal increase.
Choosing consistent inputs
Use the same index series, geographic coverage, seasonal-adjustment basis, and reference base for both periods. Do not mix annual averages with monthly observations without considering the comparison you intend to make. Enter index levels, not annual percentage inflation rates.
For US consumer-price data, consult the Bureau of Labor Statistics Consumer Price Index. Record the series and periods you choose; the calculator’s prefilled indexes are not labeled historical observations.
Limits of this comparison
Cumulative inflation covers the entire interval between your index values, not an annualized rate. A household’s actual expenses can change differently from a broad price basket. This calculation does not include interest, taxes, salary changes, or future inflation.
A starting index must be positive for a meaningful ratio. With zero or a negative starting index, this tool displays fallback zeros rather than a valid economic comparison. Rounded results should not be treated as exact price quotes.