An inflation calculator shows what today's price will become after years of rising prices. At 3% a year, a $250 weekly grocery bill turns into $335.98 in 10 years. That's 34.4% more, not 30%.
Here's where the extra comes from, how to estimate when prices double, and how the official US figures are measured.
Why 3% a year adds up to more than 30%
Each year's rise lands on a price that has already gone up. That's compounding. It's why cumulative inflation, the total rise over the period, exceeds the simple sum of the yearly rates:
On a $100 price, the dollar increase equals cumulative inflation in percent. Here's 3% a year over five periods:
| Period (simple sum of 3% a year) | Cost of a $100 item | Cumulative inflation |
|---|---|---|
| 1 year (3%) | $103.00 | 3.00% |
| 5 years (15%) | $115.93 | 15.93% |
| 10 years (30%) | $134.39 | 34.39% |
| 20 years (60%) | $180.61 | 80.61% |
| 30 years (90%) | $242.73 | 142.73% |
For 30 years, the simple sum says 90%. The true rise is 142.73%, so prices end up almost two and a half times higher. Over a single year the two agree, and the gap widens every year after that.
The rule of 70: how long until prices double?
For a quick estimate, divide 70 by the yearly inflation rate. The result is roughly the number of years prices take to double. At 2%, that's 35 years; at 7%, about 10. The shortcut works because the natural logarithm of 2 is about 0.693, and 70 is close to 100 times that.
Here the calculator checks the rule on a $100 price:
| Inflation rate | Years by the rule (70 ÷ rate) | $100 becomes |
|---|---|---|
| 1% | 70 yr | $200.68 |
| 2% | 35 yr | $199.99 |
| 3.5% | 20 yr | $198.98 |
| 5% | 14 yr | $197.99 |
| 7% | 10 yr | $196.72 |
| 10% | 7 yr | $194.87 |
Up to 5%, the rule lands within about $2 of $200. At 10%, prices need a little over 7 years to double, so the rule's answer is slightly short. When prices double, a dollar buys half as much, which the buying power calculator shows directly. For savings and investments, the rule of 72 calculator applies the same idea with 72.
How the BLS measures consumer prices
The most widely used US inflation measure is the consumer price index (CPI), from the Bureau of Labor Statistics (BLS). It tracks the average change in prices consumers pay for a representative basket of goods and services.
Collecting the prices
Each month, BLS staff record the prices of about 80,000 items. They gather them in 75 urban areas, from about 6,000 housing units and some 22,000 retail establishments. Collection happens by visit, phone call, web or app.
Weighting the basket
The BLS sorts spending into more than 200 categories within eight major groups. Food and beverages, housing and medical care are three of those groups. The weights come from the Consumer Expenditure Surveys, in which households report what they actually bought. Each year, with the January index, the weights switch to the most recent spending data.
Combining the prices
First, the BLS builds a basic index for each of 7,776 item-and-area combinations, mostly with a weighted geometric mean. Then it averages those into broader indexes, using the spending weights. The headline CPI-U, the index for all urban consumers, covers over 90% of the US population.
Which inflation rate should you use?
The newest BLS reading, released September 11, 2026, puts the CPI-U up 3.4% over the 12 months to August. That's before seasonal adjustment. Without food and energy, the rise was 2.4%. Energy prices jumped 16.3%.
The yearly pace moves, too. It was 4.2% for the 12 months to May 2026 and 3.5% to June. It then held at 3.4% in July and August. In June, the seasonally adjusted index even fell 0.4%, its largest one-month drop since April 2020.
The Federal Reserve's 2% goal for the longer run uses another gauge, the price index for personal consumption expenditures (PCE). For next year's budget, start from the latest CPI-U figure. For a 20- or 30-year plan, test a range instead, such as 2% and 4%.
Your household's inflation isn't the headline rate
The BLS says plainly that the CPI may not match your own experience. It prices the basket of an average urban household, not any particular family. So your mix of spending sets your personal rate:
Extra cost on $100 of spending after a year
By CPI-U category, 12 months to August 2026, not seasonally adjusted (BLS)
A budget heavy in gasoline ran above 3.4%. At the energy index's pace, a $300 monthly energy bill would now be $348.90. Medical care prices, by contrast, rose just 1.6%. If your spending leans toward the fastest-rising categories, enter a higher rate than the headline.
Measuring inflation between two dates
The calculator projects forward at one steady rate. To measure what already happened, compare two CPI readings with the BLS percent-change formula. The second line turns the result into an average yearly rate you can enter here:
The CPI-U read 333.952 in June 2026 and 334.980 in August, both on a 1982–84 base of 100. That's a rise of 0.31% in two months: 1.028 ÷ 333.952 × 100. The CPI inflation calculator works out this percent change for any two readings.
Over longer spans, use the second line. Say an index rose from 250 to 300 in 5 years: (300 ÷ 250)^(1 ÷ 5) − 1 is about 3.71% a year. Entered here with $250 and 5 years, that rate returns $299.95. For escalating a contract or payment, the BLS strongly recommends indexes that aren't seasonally adjusted.
Sources
- U.S. Bureau of Labor Statistics, Consumer Price Index – August 2026, with technical note (news release, September 11, 2026)
- U.S. Bureau of Labor Statistics, Consumer Price Index – June 2026 (news release, July 14, 2026)
- U.S. Bureau of Labor Statistics, Consumer Price Index Frequently Asked Questions (last modified September 25, 2025)
- U.S. Bureau of Labor Statistics, Handbook of Methods: Consumer Price Index, Calculation (accessed September 26, 2026)
- Federal Reserve, Why does the Federal Reserve aim for inflation of 2 percent over the longer run? (updated August 22, 2025)