Monthly and annual inflation rates can move in different directions because they compare prices across different windows. A monthly CPI change tracks one month against the previous month, with seasonal adjustment commonly used to assess short-term trends. The 12-month rate compares the index with its level a year earlier. In September 2026, consumer prices rose 0.6 percent from August on a seasonally adjusted basis, while the unadjusted index was 3.7 percent higher than a year earlier Bureau of Labor Statistics.
Two windows, two questions
The Consumer Price Index measures average changes over time in prices paid by consumers for a representative basket of goods and services. It is a weighted average: price changes count according to how important those items are in the spending patterns of the population represented Bureau of Labor Statistics. That makes the CPI useful for describing broad price movement, but it does not mean every household faces the same change.
A one-month rate asks what shifted recently. A 12-month rate asks how the index now compares with the same month last year. These are not competing answers. The first can reveal a fresh acceleration or pause; the second shows the cumulative change across a longer period. The CPI release publishes both monthly and 12-month percentage changes, and the seasonal-adjustment status differs in the September headline figures Bureau of Labor Statistics Bureau of Labor Statistics.
Why seasonal adjustment matters
Some prices tend to move at particular times of year because of weather, production cycles, model changeovers, holidays or sales. BLS seasonal adjustment estimates and removes recurring patterns so analysts can better assess whether a monthly change is unusual for that time of year Bureau of Labor Statistics. The adjustment is a statistical treatment of the series; it does not change the prices consumers actually paid.
That distinction explains why September’s all-items CPI rose 0.6 percent on a seasonally adjusted monthly basis, while the release reports the 12-month increase as 3.7 percent before seasonal adjustment Bureau of Labor Statistics. Unadjusted data are often more relevant to people looking at actual prices and are used in some escalation arrangements. Seasonally adjusted CPI series can be revised annually, which is why BLS cautions against using them in such arrangements Bureau of Labor Statistics.
A spike can lift one reading more than another
An annual rate is built from the price level across a full year, so one new monthly jump usually has a smaller effect on that comparison than on the one-month rate. The 12-month figure also reflects what happened in earlier months and which month from the prior year drops out of the comparison. If a month with a large increase replaces a month with a small increase, the annual rate can rise even if the latest monthly movement is modest.
September offers a concrete example of a sharp monthly contribution: gasoline prices rose 9.1 percent from August on a seasonally adjusted basis, accounting for more than half of that month’s all-items increase. The energy index rose 20.4 percent over the year, while food rose 2.5 percent Bureau of Labor Statistics. Those figures show why looking only at the headline can conceal very different category patterns; they do not establish how any particular household’s expenses changed.
Persistence is different from a one-off jump
A single month of faster inflation can push up the monthly rate without proving that price increases are persistently accelerating. If comparable increases continue month after month, their effects accumulate and remain in the 12-month window for longer. Conversely, a brief spike can fade from the annual comparison after it passes out of the 12-month period, even if prices remain higher than before the spike.
For context, BEA’s separate PCE price index rose 0.5 percent monthly and 3.6 percent over the year in September Bureau of Economic Analysis. PCE and CPI are different indexes, with different construction and coverage, so their figures should not be treated as interchangeable measures of the same calculation. Reading a rate carefully means checking the index, window and adjustment status first, then asking whether the change reflects one sharp move or a sustained sequence.
Sources & further reading
- Consumer Price Index questions and answers ↗Bureau of Labor Statistics
- September 2026 Consumer Price Index ↗Bureau of Labor Statistics
- September 2026 personal income and outlays ↗Bureau of Economic Analysis
