Seasonal adjustment is meant to make month-to-month price data easier to compare by accounting for patterns that tend to recur at the same time each year. It does not change what households paid, and it does not turn a monthly movement into an annual rate. In the September 2026 Consumer Price Index, the all-items measure rose 0.6 percent from August after seasonal adjustment, while the unadjusted index was 3.7 percent higher than a year earlier. Those figures describe different spans of time and should be read separately. Bureau of Labor Statistics
A calendar pattern can obscure a new move
Prices often move in recognizable seasonal ways. Weather can affect food and fuel, holidays can change shopping patterns, and product cycles or sales can shift prices at predictable points in the year. Seasonal adjustment estimates those recurring patterns so analysts can better see whether a particular month’s movement departs from the usual calendar rhythm. Bureau of Labor Statistics
The adjustment is statistical, not a discount or a correction to a shopper’s receipt. BLS produces adjusted and unadjusted CPI measures using seasonal factors, which are updated each February; seasonally adjusted series can be revised for up to five years. The adjusted series is therefore useful for short-term analysis, while its revision schedule matters when a figure is being used in a long-lived agreement. Bureau of Labor Statistics
September’s two figures cover different periods
In the September 2026 release, the CPI for All Urban Consumers rose 0.6 percent from the preceding month on a seasonally adjusted basis. The unadjusted all-items index rose 3.7 percent over the 12 months ending in September. The first compares September with August after seasonal adjustment; the second compares the index level with September a year earlier without seasonal adjustment. Bureau of Labor Statistics
Calling both figures “inflation” without naming their periods can invite a false comparison. The 0.6 percent monthly figure is not a smaller version of the 3.7 percent annual figure. Nor should the monthly change simply be multiplied by 12 and treated as the year’s measured increase: that would assume the same pace repeated every month, which the reported observation does not establish.
Adjustment does not explain every price change
The release illustrates how one component can move the headline monthly result. Gasoline rose 9.1 percent in September on a seasonally adjusted basis and accounted for more than half of the monthly all-items increase. Shelter rose 0.4 percent, while food was unchanged. These component readings help describe the mix behind the aggregate, but they do not tell any particular household exactly how its expenses changed. Bureau of Labor Statistics
CPI measures average price change for a representative basket of goods and services for its target population. Individual households can face different changes because their spending differs; a renter, a driver, and a household that spends more on medical care do not buy the same mix. Seasonal adjustment addresses recurring timing patterns in the index, not those differences in household budgets. Bureau of Labor Statistics
Choose the measure that fits the question
For a question about whether prices are accelerating or easing from one month to the next, the seasonally adjusted monthly change is generally the more useful signal. For a question about how the price level compares with the same month last year, the unadjusted 12-month change provides that comparison. For a contract or benefit tied to CPI, the relevant agreement’s specified series and treatment matter; BLS advises against using adjusted series for escalation agreements because seasonal factors are revised annually. Bureau of Labor Statistics
A practical reading of September’s report is therefore: the broad urban-consumer price index rose 0.6 percent from August after adjustment, and it stood 3.7 percent above September 2025 before adjustment. The figures can sit side by side without contradiction. One filters recurring seasonal patterns in a short comparison; the other reports the accumulated year-over-year change in the unadjusted index. Bureau of Labor Statistics
Sources & further reading
- September 2026 Consumer Price Index ↗Bureau of Labor Statistics
- Consumer Price Index questions and answers ↗Bureau of Labor Statistics
