BLS documentation describes the building blocks of the Consumer Price Index (CPI). Prices are collected each month in 75 urban areas. The urban areas of the United States are divided into 32 index areas, and consumer goods and services are divided into 211 item strata. Since January 1999, most basic indexes within the CPI have been calculated with a geometric mean formula. The sources reviewed here describe these pieces, but they do not lay out every arithmetic step from a single collected price to the headline inflation rate. This explainer separates what the sources state from our own reading and shows where the evidence stops.
How CPI price data are collected
According to the BLS CPI technical notes, CPI prices are collected each month in 75 urban areas from about 6,000 housing units and approximately 22,000 retail establishments. The wording matters. BLS says "about" and "approximately," so these are rounded descriptions of the sample, not exact counts.
Collection methods have changed for some items. The BLS CPI questions and answers page states two such changes:
- Gasoline: Beginning with the release of June 2021 indexes, BLS uses a secondary source dataset for gasoline instead of traditional in-person or website collection.
- New vehicles: With the release of April 2022 indexes, BLS-collected data for the new vehicles index was replaced with transaction data from J.D. Power.
Our interpretation: "collecting prices" does not mean one uniform method for every item. The sources name these two categories. They do not give a full list of which other items, if any, use secondary or transaction data, so we do not extend the point beyond gasoline and new vehicles.
How the CPI is organized by area and item
The BLS Handbook of Methods chapter on CPI calculation describes two dimensions that structure the index: geography and items.
| Dimension | What the BLS handbook states |
|---|---|
| Geography | The urban areas of the United States are divided into 32 geographic areas called index areas. |
| Items | Consumer goods and services are divided into 211 item strata: 209 commodities and services item strata and 2 housing item strata. |
| Combinations | The CPI has 7,776 (32 x 243) item-area combinations. |
One detail needs care. The handbook gives 211 item strata, but the multiplication behind the 7,776 item-area combinations uses 243, not 211. The claims we rely on do not explain how 243 relates to the 211 item strata. Rather than guess, we leave that question open. Readers who want the full breakdown should read the handbook chapter itself.
Our reading of the structure: to calculate CPI inflation, BLS works with a grid of items crossed with areas, not one national list of prices. The sources reviewed here do not spell out how each cell of that grid is weighted or combined into higher-level indexes.
How basic CPI indexes are calculated
The formula is the part of the question the evidence covers least. The BLS CPI questions and answers page states that since January 1999, a geometric mean formula has been used to calculate most basic indexes within the CPI.
Note the word "most." The source does not say every basic index uses a geometric mean, and the claims reviewed here do not identify which basic indexes use a different formula, or what that formula is.
The BLS CPI technical notes add two facts that help with reading the results:
- Reference base: The reference base for the C-CPI-U is December 1999 equals 100. An index level is read relative to that base.
- Statistical uncertainty: The estimated standard error of the 1-month percent change for the U.S. all items CPI is 0.04 percent.
Our interpretation of the standard error: a monthly CPI change is a statistical estimate from a sample, not an exact measure of every price in the economy. The 0.04 percent figure is itself labeled an estimate, and it applies to the 1-month percent change for the U.S. all items CPI. The claims do not give standard errors for other periods, areas or categories, so we do not apply that figure to them.
Where CPI inflation data are published
The BLS Handbook of Methods chapter on CPI concepts describes how far the published data reach:
- CPI data are published for 23 metropolitan areas, 4 geographic regions and 9 divisions as defined by the Census Bureau.
- Seasonally adjusted CPI data are published only at the nationwide level, and not all categories are seasonally adjusted.
- Tariffs are not explicitly included in the CPI.
These points affect how CPI inflation figures can be read. Our reading: if you look up a figure for a metropolitan area or region, it will not be seasonally adjusted, because seasonal adjustment is published only nationwide. Also, a category at the national level is not necessarily seasonally adjusted.
On tariffs, the source says only that they are not explicitly included. The claims we rely on do not say whether or how tariffs may appear indirectly in collected prices, so this article makes no statement either way.
Who the CPI covers and affects
The BLS CPI questions and answers page gives two facts about scope and use:
- The CPI-U population makes up over 90 percent of the U.S. population.
- Because of statutory action, the CPI affects the income of more than 108 million people, including over 67 million Social Security beneficiaries and over 41 million SNAP recipients.
These are lower bounds ("over" and "more than"), not exact totals. The source links the CPI's effect on income to statutory action. The claims reviewed here do not describe the specific laws or adjustment formulas involved, so we do not describe them either.
What the evidence does not show about CPI calculation
This section is our interpretation, based on the BLS Handbook of Methods chapter on CPI calculation and the other BLS pages cited above.
What the sources do establish:
- Prices come from monthly collection in 75 urban areas, plus secondary source data for gasoline and J.D. Power transaction data for new vehicles.
- The index is organized into 32 index areas and 211 item strata, with 7,776 item-area combinations.
- Most basic indexes have used a geometric mean formula since January 1999.
- The 1-month percent change for the U.S. all items CPI has an estimated standard error of 0.04 percent.
What the reviewed claims leave unresolved:
- The full formula for combining basic indexes into the all items index.
- How item and area weights are set or updated.
- Which basic indexes do not use the geometric mean formula.
- How the 243 in the item-area calculation relates to the 211 item strata.
- How a monthly or annual inflation rate is derived from index levels in the published releases.
Readers who need these details should go to the BLS documentation directly instead of relying on a summary.
Limits of this article
This article is educational information about how BLS describes the Consumer Price Index. It is not investment, tax or legal advice. It does not cover the full weighting method, every formula or how to use CPI figures in personal financial decisions.