data / Field guide
How to read a CPI release
A field guide to monthly, twelve-month, adjusted, unadjusted, headline, and component figures in the same release.
Read the series label and comparison window before the number. CPI releases commonly pair a seasonally adjusted one-month change with an unadjusted twelve-month change; those figures answer different questions and should not be treated as conflicting versions of one statistic.
Read the first paragraph twice
The opening of a CPI release usually presents at least two time windows. The one-month change describes movement from the preceding month. The twelve-month change compares the current month with the same month a year earlier. BLS often reports the monthly all-items change on a seasonally adjusted basis and the twelve-month change before seasonal adjustment.
Those qualifiers are part of the number. Seasonal adjustment aims to remove recurring seasonal patterns so short-term movement is easier to inspect. An unadjusted twelve-month comparison uses the same calendar month at both ends, which naturally controls for many seasonal patterns. Neither window is automatically more honest; each answers a different question.
- Monthly change: what happened since the immediately preceding month?
- Twelve-month change: how different is the index from the same month one year earlier?
- Annual average change: how did the average of twelve monthly index levels compare with another year’s average?
Sources: [bls-faq] [bls-percent]
Find the exact series before interpreting it
CPI is a family of indexes. CPI-U covers all urban consumers as defined by BLS; CPI-W covers urban wage earners and clerical workers; chained CPI has a different construction and revision pattern. Releases also include all-items measures, indexes for major categories, and all items less food and energy. A number detached from its series label can easily be assigned the wrong meaning.
The index level is not a dollar price. Most CPI series use a reference base, such as 1982–84 equals 100, so the level expresses change relative to that base. BLS recommends percent changes for comparing movements because an index-point change depends on the level of the index. Do not read a level of 300 as a 300 percent monthly or annual increase.
- Record population, geography, item category, and reference base.
- Separate all items from “core” shorthand for all items less food and energy.
- Check footnotes; some component values in a table may use a different adjustment convention.
Sources: [bls-faq]
Use the tables to test the narrative
The summary names categories that contributed to the latest movement, but the tables provide the structure needed to verify it. Table A in a typical release places several monthly seasonally adjusted changes beside the latest unadjusted twelve-month change. Additional tables provide index levels and more detailed categories. Trace any quoted figure to its row and column rather than relying on a cropped screenshot.
A large move in one category can shape the monthly total without defining the longer trend. Conversely, a category may fall in the latest month while remaining above its year-earlier level. Contribution language also differs from category change: saying a category accounted for much of the all-items movement is not the same as saying it had the largest percentage increase.
- Match the quoted number to its row, column, units, and adjustment label.
- Distinguish a category’s own percent change from its contribution to the aggregate.
- Look across several months before calling one monthly movement a trend.
Sources: [bls-faq]
Reproduce a percent change carefully
BLS gives the standard calculation: subtract the earlier index from the later index, divide by the earlier index, and multiply by 100. A valid twelve-month calculation uses the same month in consecutive years. January to December covers eleven month-to-month intervals, so BLS warns that it is not a twelve-month change.
Use index levels from the same series and the same reference base. Expect a small difference when reproducing a published rate from displayed levels because agencies may calculate with more precision than the rounded values shown. Also note whether a series can be revised. BLS explains that seasonally adjusted CPI indexes may be revised when seasonal factors are updated, while many unadjusted CPI indexes are not revised after publication.
- Formula: ((later index − earlier index) ÷ earlier index) × 100.
- Do not add rounded monthly rates to claim an exact annual rate.
- Preserve the release date or vintage when documenting a value that may later be revised.
Sources: [bls-percent] [bls-faq]
Primary sources
- Consumer Price Index Frequently Asked QuestionsU.S. Bureau of Labor Statistics · accessed 2026-10-19
- Calculating percent changesU.S. Bureau of Labor Statistics · accessed 2026-10-19
Prepared with AI assistance and reviewed against the linked primary sources. This is public financial education, not personal financial or investment advice.
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