Inflation in the Consumer Price Index is calculated as the percentage change in the index between two periods. The index tracks the average change over time in prices urban consumers pay for a representative basket of goods and services, according to the U.S.
Bureau of Labor Statistics' CPI calculation handbook. The calculation combines thousands of observed prices, but it does not treat every purchase equally. Goods and services that account for more consumer spending receive greater weight.
Table of Contents
- Turning index levels into an inflation rate
- What goes into the CPI basket
- How thousands of prices become one index
- How quality and quantity changes are handled
- Which CPI number should readers use?
Turning index levels into an inflation rate
The CPI is a price index, not an inflation rate by itself. Inflation is the percentage change between two index levels, as explained in the BLS guide to CPI concepts.
For example, suppose the index rises from 300 to 309. The calculation is: The same method works for monthly or yearly inflation. The periods must match the question: A one-month change and a 12-month change describe different time spans.
- Subtract the earlier index from the later one: 309 minus 300 equals 9.
- Divide the difference by the earlier index: 9 divided by 300 equals 0.03.
- Multiply by 100: The inflation rate is 3%.
What goes into the CPI basket
BLS draws connected samples of urban areas, stores and other outlets, particular goods and services, and rental housing. It collects prices throughout the month rather than relying on a single-day snapshot. The agency then weights price changes according to spending patterns. A category with a larger share of consumer expenditures has more influence on the overall index than a category with a smaller share.
BLS has updated spending weights annually since the January 2023 index. Those weights use expenditure data from two years earlier on average, according to its relative-importance and weight information. That lag matters when buying habits change quickly. Consumers may shift away from items whose prices rise faster, while CPI weights take time to reflect the new spending pattern.
How thousands of prices become one index
The calculation happens in stages. BLS first creates basic indexes for particular items within particular geographic areas. For most goods and services, it combines weighted price relatives—the current price divided by the previous price—using a geometric-mean formula. A limited number of categories use a modified Laspeyres formula instead.
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BLS then combines the basic item-area indexes into published measures such as CPI-U and CPI-W. This stage uses modified Laspeyres aggregation, giving greater influence to categories that represent more spending in the Consumer Expenditure Survey. This structure explains why a dramatic price increase in a small category may barely move the overall CPI. A modest increase in a heavily weighted category can have a much larger effect.
How quality and quantity changes are handled
A higher price does not always represent pure inflation. A replacement product may offer different features, or a package may contain a different quantity. BLS attempts to measure constant-quality price change by adjusting for those differences.
In some categories, it uses hedonic regressions—statistical estimates of the value associated with product characteristics—rather than counting an improved product's entire price increase as inflation, as described in the agency's quality-adjustment guidance. These adjustments do not mean BLS ignores price increases. They separate the estimated value of a changed product from the remaining price change so the index compares more equivalent items.
Which CPI number should readers use?
Headline CPI figures may be unadjusted or, for selected national series, seasonally adjusted. Seasonal adjustment removes recurring effects associated with weather, holidays, sales, and production cycles. Seasonally adjusted changes are generally more useful for examining short-term trends. Readers comparing figures should check that both numbers use the same adjustment and cover the same period.
CPI-U covers more than 90% of the U.S. population, but it is not every household's personal inflation rate. It excludes rural nonmetropolitan and farm households, people living on military installations, and institutionalized people. Individual experiences also depend on spending. A household devoting an unusually large share of its budget to a fast-rising category can face higher inflation than the national average, while another household may face less.
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