A plain-language guide to the Consumer Price Index: what each number on a CPI release means, how the Bureau of Labor Statistics builds the index from a fixed market basket of goods and services, and how that index becomes the inflation rate you see in the news.
How do I read CPI numbers?
CPI is published as an index number, not a dollar price. The BLS sets the 1982–1984 average to 100, so a CPI-U of 320 means prices are about 3.2 times their early-1980s level. The three numbers worth reading on any release are the index level itself, the year-over-year change (this month's index versus the same month a year earlier, the headline 'inflation rate') and the month-over-month change (this month versus last month, which shows short-term direction). A rising index means prices are increasing; a falling index would mean broad deflation, which is rare.
What is the difference between the index level and the inflation rate?
The index level is the absolute number the BLS publishes for a given month (for example 320.0). The inflation rate is a percentage change computed from two index levels. Year-over-year inflation is (index this month − index same month last year) ÷ index same month last year × 100. Month-over-month is the same formula against the prior month. The index level keeps climbing as long as prices rise; the inflation rate can fall even while the index keeps rising — it just rises more slowly.
How is the CPI calculated?
The BLS builds the CPI from a fixed market basket of goods and services that represents what urban consumers actually buy — food, housing, transportation, medical care, education, recreation and more. Each item is tracked at a set of sample outlets in cities across the country. The BLS collects prices every month, weights each category by its share of consumer spending (shelter gets the largest weight), and combines the category indexes into the all-items index using a Laspeyres-style formula that holds the basket composition fixed between major rebasings. The published index is the cost of that fixed basket relative to the 1982–1984 base period.
What is the CPI market basket?
The basket has eight major groups: food and beverages, housing, apparel, transportation, medical care, recreation, education and communication, and other goods and services. Within housing, 'shelter' (rent and owners' equivalent rent) is by far the largest single weight in the CPI — roughly a third of the entire index. Weights come from the Consumer Expenditure Surveys and are updated to reflect how spending patterns change over time.
What is the difference between CPI-U, CPI-W and Chained CPI?
CPI-U covers all urban consumers and is the headline number quoted in the news. CPI-W covers urban wage earners and clerical workers and is the series used for Social Security cost-of-living adjustments. Chained C-CPI-U allows for households substituting between goods as relative prices change, so it usually rises slightly more slowly than CPI-U. All three are published by the BLS from the same underlying price data.
What is Core CPI and why does it matter?
Core CPI strips out food and energy, the two most volatile categories, so it shows the underlying trend in inflation rather than swings driven by gas or grocery prices. Food and energy can jump or fall sharply from month to month for reasons unrelated to broad price pressures. Because core removes that noise, the Federal Reserve watches it closely as a signal of where inflation is actually heading.
Is the CPI adjusted for quality changes?
Yes. When a product improves (a car with better safety features, a phone with more capability) or changes in size, the BLS applies quality adjustments so the index reflects pure price change rather than the value of the improvement. This is one reason the CPI is a measure of the cost of maintaining a constant standard of living, not literally the price of identical physical goods over decades.
How often is the CPI updated?
The BLS releases the CPI once a month, usually mid-month at 8:30 a.m. Eastern, covering the previous month. CPI Monitor re-checks the published series automatically, so the index level and inflation rate on this site change on their own when a new release lands. The underlying basket weights are refreshed periodically to keep pace with how consumers actually spend.