An inflation rate turns a change in a price index into a percentage over a specified span. The arithmetic is simple; choosing the span and understanding what the index represents take more care. The Consumer Price Index (CPI) tracks average price changes for a representative basket of goods and services purchased by urban consumers. A one-month rate describes recent movement, while a 12-month rate compares today’s index with the same month a year earlier. Those comparisons can tell different stories when prices rise unevenly.
From a basket to an index
The CPI does not simply add up every price shoppers encounter. The Bureau of Labor Statistics groups consumer purchases into categories, samples prices for selected goods and services, and weights price changes according to their importance in spending. The basket includes items such as food, housing, clothing, transport and medical care; investment assets such as stocks and bonds are outside its day-to-day consumption measure. The result is an index that summarizes price movement for a defined population, rather than a bill for a particular household. Bureau of Labor Statistics
Imagine, hypothetically, a tiny basket containing only bread and bus rides. If bread accounts for three-quarters of its spending and bus rides one-quarter, a 4% bread-price increase and a 0% bus-fare change would produce about a 3% basket increase, assuming the weights and quantities stay fixed. Actual CPI construction is more elaborate, with many categories and statistical procedures, but the example shows why a large rise in a frequently purchased item can matter more than a similar rise in a rarely purchased one.
The calculation is a ratio
To calculate a percent change, subtract the earlier index from the later one, divide that difference by the earlier index, then multiply by 100. If a hypothetical index moves from 200 to 204, the increase is 4 index points, or 2%: (204 − 200) ÷ 200 × 100. The percentage, rather than the point difference, makes changes easier to compare across index levels. An index’s reference base sets its scale; the rate comes from comparing two observations, not from the number printed as the index level. Bureau of Labor Statistics
This is a rate of change in the index, not a claim that every price rose by that percentage. Some prices can fall while others climb, and a weighted average can still rise. A household that spends unusually heavily on gasoline, rent or medicine may see its own costs move differently from the national average. BLS says the CPI reflects an average household experience and does not necessarily measure any individual’s inflation. Bureau of Labor Statistics
Monthly and annual rates answer different questions
A monthly rate compares one month with the immediately preceding month. For September 2026, the CPI for All Urban Consumers rose 0.6% on a seasonally adjusted basis from August. The 12-month rate compares September’s index with September 2025; the all-items CPI rose 3.7% over that span, measured before seasonal adjustment. The first figure captures the latest month’s movement after seasonal patterns are accounted for. The second describes the cumulative change across a year and includes the comparison month’s position in that period. Bureau of Labor Statistics
A monthly change is not simply the yearly rate divided by 12. Prices do not move at a constant pace: the September release reported gasoline up 9.1% in the month, while food was unchanged and shelter rose 0.4%. Those component movements help explain why one month can look different from a yearlong average. Seasonally adjusted monthly figures are often useful for reading short-term trends; unadjusted figures reflect the prices consumers actually encounter and are commonly used in escalation arrangements. Bureau of Labor Statistics
The comparison month matters
A 12-month rate depends on both the latest month and the month that drops out of the comparison. If the earlier year had an unusually large jump, the annual rate can ease even when prices are still rising now. If the earlier month was unusually flat, the annual rate can increase without a sudden acceleration in the latest month. This is why a year-over-year rate is not a forecast, and why a single monthly reading does not establish a lasting trend.
The CPI is also one price measure among several. The Personal Consumption Expenditures price index covers prices of goods and services purchased by people in the United States or on their behalf, and it is known for reflecting a wide range of expenses and shifts in consumer behavior. Its September 2026 12-month change was 3.6%, while September CPI inflation was 3.7%; they are different indexes with different coverage and methods, so the figures should not be treated as interchangeable. Bureau of Economic Analysis
Sources & further reading
- Consumer Price Index questions and answers ↗Bureau of Labor Statistics
- Personal consumption expenditures price index ↗Bureau of Economic Analysis
- September 2026 Consumer Price Index ↗Bureau of Labor Statistics
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