What Is CPI? Definition, Formula, and Common Misreads

The Consumer Price Index (CPI) is a monthly measure of the average change over time in the prices paid by urban consumers for a fixed basket of goods and services. Published by the U.S. Bureau of Labor Statistics (BLS), it is the most widely cited gauge of inflation. When people ask "what is CPI," they usually want to know what it actually measures, how it is calculated, and why the headline number can differ from their own experience. This page answers those questions and explains the most common ways the index is misread.

For a focused look at the year-over-year change and how to read it, see CPI Year over Year.

What the CPI basket contains

The CPI is built from a market basket that represents the spending of urban consumers. The basket is divided into eight major groups: food and beverages, housing, apparel, transportation, medical care, recreation, education and communication, and other goods and services. Each group has a weight that reflects its share of total consumer spending. Those weights are updated periodically based on the Consumer Expenditure Survey, so the basket evolves with spending patterns.

Importantly, the CPI measures price changes for a fixed basket. It does not measure the cost of living for any specific household. Your personal inflation rate can differ from the CPI because your spending mix, location, and shopping habits may not match the national average.

How CPI is calculated

The BLS uses a modified Laspeyres formula. In simple terms, the index compares the cost of the current basket at current prices to the cost of the same basket at base-period prices. The general formula is:

CPI = (Cost of basket in current period / Cost of basket in base period) x 100

The base period is currently 1982-84, meaning the index is set to 100 for the average price level in that period. The calculation involves collecting prices for thousands of items each month from retail outlets, service establishments, and housing units across the country. The BLS then aggregates these prices using the item weights to produce the all-items index.

Two variants are commonly reported: - CPI-U: All Urban Consumers, covering about 93% of the U.S. population. This is the headline number. - CPI-W: Urban Wage Earners and Clerical Workers, used for adjusting某些 government benefits and tax brackets.

The BLS also publishes a chained CPI (C-CPI-U), which uses a Törnqvist formula to account for substitution between items as relative prices change. The headline CPI does not allow for this substitution, so it can slightly overstate inflation compared to the chained measure.

Release schedule and lag

The CPI is released monthly by the BLS, typically in the middle of the month following the reference month. For example, the January CPI is usually published around the middle of February. The exact date is announced in advance on the BLS website. The release covers price data collected during the entire reference month, so there is a lag of roughly two to three weeks between the end of the month and the publication date.

Because the data are seasonally adjusted for some series, the BLS also publishes unadjusted figures. The headline year-over-year change is usually calculated from unadjusted data, while month-to-month changes are often discussed on a seasonally adjusted basis.

Common misreads

Even though CPI is a straightforward concept, several misreadings are common:

1. Confusing the index level with the inflation rate. The CPI itself is an index number (for example, 300), not a percentage. The inflation rate is the percentage change in the index over a period. A headline like "CPI rose 3%" refers to the year-over-year change, not the index level.

2. Ignoring the difference between headline and core. Core CPI excludes food and energy because those prices are volatile. Headline CPI includes everything. Neither is "right" or "wrong"; they answer different questions. For a detailed treatment of core CPI, see the dedicated page.

3. Assuming CPI measures the price of everything. CPI covers only out-of-pocket spending by urban consumers. It excludes investment assets, business purchases, and prices paid by rural households. It also does not include income taxes.

4. Treating a single month's move as a trend. Monthly CPI changes can be noisy. The BLS publishes seasonally adjusted figures to remove regular seasonal patterns, but one month can still be affected by idiosyncratic factors. Year-over-year changes are less noisy but respond slowly to turning points.

5. Expecting CPI to match your personal experience. Your spending basket likely differs from the national average. For example, if you spend a larger share on food or rent than the average urban consumer, your personal inflation rate may be higher or lower than the CPI.

Table: CPI variants and what they capture

What CPI does and does not tell you

CPI tells you how the average price of a fixed basket of consumer goods and services has changed over time. It is used to adjust wages, benefits, and tax brackets for inflation, and it is a key input for the Federal Reserve's inflation goals. However, CPI does not tell you whether inflation is high or low in absolute terms; that judgment depends on context and historical comparison. It also does not forecast future inflation. It is a backward-looking measure of price changes that have already occurred.

For a broader view of inflation measures, including the Personal Consumption Expenditures (PCE) price index, see the PCE Inflation page. To understand how inflation expectations are formed, the Inflation Expectations page provides context.

When this does not apply

This explanation of CPI applies to the U.S. Consumer Price Index as published by the BLS. It does not apply to other countries' consumer price indices, which may use different baskets, formulas, and methodologies. It also does not cover producer price indices (PPI) or import/export price indices, which measure prices at different stages of production or trade. Finally, if you are looking for a real-time measure of inflation or a forecast, CPI is not designed for that purpose; it is a retrospective statistical estimate.

FAQ

What is CPI in simple terms?

CPI, or Consumer Price Index, is a number that tracks the average change in prices for a basket of goods and services that urban consumers buy. It is published monthly by the U.S. Bureau of Labor Statistics and is used to measure inflation.

How is CPI calculated?

The BLS uses a modified Laspeyres formula. It compares the cost of a fixed basket of goods and services at current prices to the cost of the same basket in a base period (currently 1982-84), then multiplies by 100. The result is an index level, and the percentage change in that index is the inflation rate.

What is the difference between CPI and core CPI?

CPI includes all items in the basket, while core CPI excludes food and energy because those prices tend to be volatile. Core CPI is often used to see the underlying inflation trend, while headline CPI reflects the full cost of living changes.

When is CPI released?

The CPI is released monthly by the BLS, usually in the middle of the month following the reference month. For example, January CPI data is typically published in mid-February. The exact release dates are announced in advance on the BLS website.