What Is CPI? How the Consumer Price Index Is Measured

The Consumer Price Index (CPI) is a monthly US Bureau of Labor Statistics index of the average change in prices urban consumers pay for a fixed basket. It is the most quoted inflation gauge in the United States.

This page covers what the index contains, how its basket is weighted, how to read headline and core, and how a monthly change becomes an annual rate. It also covers what CPI does not measure, and no live figure appears here. It is descriptive throughout.

What CPI measures

Each month the Bureau of Labor Statistics (BLS) collects prices on tens of thousands of items from retailers, service providers and landlords across US urban areas, and combines them into one number. The index has no natural units: its level only means something against a reference period fixed at 100, so a reading near 300 says the basket costs about three times its reference-period cost. What matters is the change.

The headline series is CPI-U, for All Urban Consumers. Other variants exist: CPI-W, for urban wage earners, sets Social Security cost-of-living adjustments, and a chained version indexes federal tax brackets. A story saying "CPI came in at" means CPI-U.

How the basket is built and weighted

Weights come from the Consumer Expenditure Survey, which asks thousands of households what they actually spend money on. Shelter, food, energy, transportation, medical care, apparel and recreation each enter in proportion to their share of it.

Shelter carries by far the largest single weight, well ahead of food and energy. It is measured through rent and owners' equivalent rent, an estimate of what an owner-occupied home would rent for. House prices and mortgage payments are not in the index.

That weighting is why felt inflation differs from the print. A renter whose rent climbs quickly, with a long commute and out-of-pocket medical bills, lives a different basket from the survey average. The index measures a national average, not any one household.

Headline, core, and the two ways of measuring change

Headline CPI covers the whole basket. Core CPI strips out food and energy, because both swing hard on weather, harvests and oil and those swings often reverse. Core reads the slower-moving part of the price level; headline is what households pay at the register.

Two comparisons run side by side:

• Month over month (MoM) compares this month with last month. Freshest, noisiest, usually quoted seasonally adjusted, since fuel and school supplies follow a calendar pattern.
• Year over year (YoY) compares this month with the same month a year earlier. Smoother, slower to react, usually quoted unadjusted, since a like-month comparison removes that pattern.

The most common beginner error is comparing one with the other. A 0.3 percent MoM print beside a 3.0 percent YoY print does not mean inflation collapsed. They measure different windows.

The arithmetic, worked through

All numbers here are invented.

Suppose the index reads 300.0 one month and 300.9 the next: 0.9 divided by 300.0 is 0.003, or 0.3 percent MoM. Now suppose twelve months each print 0.3 percent. The annual rate is not 0.3 times 12. The changes compound, and 1.003 to the twelfth power is about 1.0366, or 3.66 percent. That is close to 3.6, and the gap widens as the monthly rate rises.

What one hot month does to the annual figure depends entirely on the month it replaces. YoY is a rolling twelve-month window, so each new month pushes an old month out. If this month prints 0.5 percent and the month dropping out also printed 0.5 percent, the annual rate does not move at all. If the dropped month printed 0.1 percent, the annual rate rises by about 0.4 points, the difference between them. That is a base effect: arithmetic, not a change in the pace of prices.

Why markets watch the release

CPI feeds the Federal Reserve's assessment of price stability, half of its dual mandate. That assessment feeds the path of the policy rate, which feeds the Treasury curve, one reason the yield curve is read closely on data days. The size or direction of any move is not knowable in advance.

Markets do not wait for CPI to form a view on inflation. Breakeven rates, the gap between a nominal Treasury yield and an inflation-protected yield of the same maturity, price inflation on a forward-looking basis and move continuously. The inflation-protected leg of that pair is the real yield. CPI arrives monthly, describing a month already closed.

Kresmion's macro calendar lists the scheduled date and time of each CPI release alongside the other monthly data releases.

What CPI is not

Not a cost-of-living index in the strict sense. That would track the cost of holding living standards constant, including substitution toward cheaper goods as prices move. CPI-U uses a fixed basket refreshed on a schedule.

Not the same as PCE. The Personal Consumption Expenditures price index, from the Bureau of Economic Analysis, is the Fed's preferred measure. It weights categories differently, covers more, and allows for substitution, so PCE inflation typically prints below CPI.

Backward-looking. It describes prices already paid in a month that has ended. The unadjusted index level itself is not routinely revised, but seasonally adjusted figures are recalculated each year when BLS updates its seasonal factors, reaching back five years, so published seasonally adjusted monthly changes can shift after the fact.

Key takeaways

Frequently asked questions

What is the difference between CPI and core CPI?

Headline CPI covers the entire basket. Core CPI excludes food and energy, which are volatile enough that one month of oil or produce prices can swing the headline while saying little about the rest.

Does a high CPI print mean interest rates will go up?

No, not mechanically. CPI is one input among many into a rate decision, and markets price a rate path continuously, so any reaction depends on how the print compares with what was priced.

Why does CPI feel lower than my own cost of living?

Because CPI weights the basket to average national spending, and no household spends like the average. If much of your income goes to rent, childcare or medical care where prices moved fast, your own rate can run well above the print.

Is CPI the same as the inflation the Fed targets?

No. The Federal Reserve's 2 percent target refers to PCE inflation, published by the Bureau of Economic Analysis, which weights differently, covers more spending, adjusts for substitution, and typically prints below CPI.

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Source: US Bureau of Labor Statistics CPI methodology and release notes; Bureau of Economic Analysis PCE documentation. This page is information, not investment advice. Kresmion Research.