Hong Life CalculatorsUnderstanding the buying power calculator — concept and principle
This calculator uses a consumer price index (CPI) to compare money across years. It finds the amount with the same buying power in a later year, the value of the later amount in starting-year money, the cumulative and average yearly inflation, and the share of buying power lost. Use it to compare salaries or prices across decades, adjust a budget or understand inflation.
How it is calculated
- Same buying power later = amount × CPI_later ÷ CPI_start.
- Amount in starting-year money = amount × CPI_start ÷ CPI_later.
- Cumulative price increase = CPI_later ÷ CPI_start − 1.
- Average yearly inflation = (CPI_later ÷ CPI_start)^(1/years) − 1.
- Buying power lost = 1 − CPI_start ÷ CPI_later.
Use annual averages, or the same month in both years, from your country's official CPI.
The principle
A CPI tracks the cost of a fixed basket of goods and services bought by a typical household. If the index is 82% higher, the same basket costs 82% more, so you need 82% more money to buy what you used to. Dividing instead gives the reverse: how much a later amount was worth in earlier money. Because prices compound year after year, the average yearly rate is a geometric mean, not the cumulative change divided by the number of years.
Worked example
Default inputs: $100, CPI 172.2 (US annual average 2000) and 313.689 (2024), 24 years.
- Ratio = 313.689 ÷ 172.2 ≈ 1.8217.
- Same buying power in 2024 = 100 × 1.8217 ≈ $182.17.
- $100 in 2024 equals 100 ÷ 1.8217 ≈ $54.90 in 2000 money.
- Cumulative price increase ≈ 82.2%; average yearly inflation = 1.8217^(1/24) − 1 ≈ 2.53%.
- Buying power lost = 1 − 1 ÷ 1.8217 ≈ 45.1%.
| Price index (CPI) in the starting year | Same buying power in the later year |
|---|---|
| 43 | $728.66 |
| 86.1 | $364.33 |
| 129 | $242.89 |
| 172 | $182.17 |
| 215 | $145.73 |
| 258 | $121.44 |
| 344 | $91.08 |
How to read the result
"Same buying power" answers "what would I need today to match that old amount?". The buying power lost shows how much a fixed sum has shrunk. An index describes an average household; if you spend more on things that rose faster, such as rent or tuition, your personal inflation may be higher.
Common mistakes
- Mixing index bases or countries (for example US CPI with Korean prices).
- Comparing one month in one year with an annual average in another.
- Dividing cumulative inflation by years to get the yearly rate.
- Treating CPI as the change in your own costs.
Frequently asked questions
How much is $100 from 2000 worth today?
Using US CPI-U annual averages, $100 in 2000 had about the buying power of $182 in 2024.
Where can I find Korea's CPI?
Statistics Korea publishes it, and it is available through KOSIS.
Why is average inflation not 82% ÷ 24?
Inflation compounds, so the yearly rate is a geometric average.
Sources: U.S. Bureau of Labor Statistics, CPI-U, U.S. city average, all items, 1982–84 = 100, annual averages · Statistics Korea, Consumer Price Index (KOSIS)