What Is the Consumer Price Index?

The Consumer Price Index Explained

Illustration of why the consumer price index is important.

Emilie Dunphy / The Balance

The consumer price index (CPI) is a monthly measurement of U.S. prices for household goods and services. It reports inflation (rising prices) and deflation (falling prices). Both can hurt a healthy economy.

The Federal Reserve monitors price changes to ensure economic growth remains stable. The Federal Reserve uses monetary policy tools to intervene if it detects too much inflation or deflation.

Definition and an Example of CPI Calculation

The CPI is the measurement used by economists for tracking price changes in a typical "basket" of goods and services that urban consumers buy.

The Bureau of Labor Statistics (BLS) computes the CPI by taking the average weighted cost of a basket of goods in a given month and dividing it by the weighted cost of the same basket the previous month. It then multiplies this percentage by 100 to get the number for the index.

The equation works like this:

CPI formula

The index shows how much prices have changed since the base year of 1982. The CPI was 273.6 in August 2021. That's how much prices have increased since the base period of 1982 to 1984 was established at roughly 100.

The BLS publishes the percentage change since last month or last year. Prices rose by 0.3% in August 2021. There was an increase of 0.5% in the index from June 2021 to July 2021. The CPI increased by 5.3% from August 2020.

  • Alternate Name: CPI for All Urban Consumers (CPI-U)
  • Acronym: CPI

The terms CPI and inflation are often used interchangeably because inflation is the percentage increase or decrease of CPI over a certain period of time.

How the CPI Works

The basket represents the prices of a cross-section of goods and services commonly bought by urban households. This cross-section represents around 93% of the U.S. population, and it factors in a sample of 14,500 families and 80,000 consumer prices.

These are the major categories in the basket and how much each contributed to the CPI in August 2021.

Consumer Price Index Categories
Group Weight
Food 14%
Energy (Incl. Gasoline) 7%
Commodities (Incl. Medication and Autos) 21%
Housing 33%
Health care 7%
Transportation  5%
Other expenses 13%
TOTAL 100%

Housing (called shelter by the BLS) is the highest weighted category within the CPI calculation. Shelter uses the concept of "owner's equivalent of primary residence" (OER), which is how much homeowners would charge to rent their home unfurnished, without utilities. The BLS surveys homeowners in multiple urban areas every year to gather this information, replacing one-sixth of the data every year.

The CPI could give a false low inflation reading due to low rents, even when home prices are high. Low rents can result from fewer renters and increased vacancies as low interest rates spur more home purchases. At the same time, housing prices could rise due to increased market activity. This is why the CPI didn't warn of asset inflation during the housing bubble of 2005.

Conversely, rising interest rates might lead to fewer buyers in the market and falling home prices. As more people compete for apartments, rents go up.

The CPI includes sales taxes. It excludes income taxes and the prices of investments, such as stocks and bonds.

The CPI measures two commodities with wild price swings: food and energy commodities (oil and gasoline). These products are traded constantly on the commodities market. Traders can bid prices up or down based on news such as wars in oil-producing countries or droughts. The CPI often reflects these price swings as a result.

The "core" CPI solves the problem of volatile food and energy prices by excluding them. The Fed has considered core CPI in the past when deciding whether to raise the fed funds rate (the interest rate for overnight loans between banks). The core CPI is useful because food, oil, and gas prices are volatile, and the Fed's tools are slow-acting.

How CPI Affects the Fed

The Fed uses the CPI to determine whether economic policies need to be modified to prevent inflation.

It uses contractionary monetary policy to slow economic growth when it recognizes that the rate of inflation is too high. It changes the fed funds rate to make loans more expensive, which tightens the money supply—the total amount of credit allowed into the market. Slowed growth and demand puts downward pressure on prices. This returns the economy to a healthy growth rate of 2% to 3% a year.

The Federal Reserve announced a change on Aug. 27, 2020, to allow a target inflation rate of more than 2% to help ensure maximum employment. Inflation growth of 2% is preferred over time, but the Fed is willing to allow higher rates if inflation has been low for a while.

The BLS publishes a handy inflation calculator you can use to plug in the dollar value for any year from 1913 to the present. It will tell you what the dollar amount is or was worth for any of those years using the average CPI for that calendar year.

How CPI Affects Other Government Agencies

The government uses the CPI to improve benefit levels for recipients of Social Security and other government programs that provide financial assistance.

How CPI Affects Housing and Investments

Landlords use the CPI forecasts to determine future rent increases in contracts.

An increased CPI can depress bond prices, too. Fixed-income investments tend to lose value during inflation. Investors demand higher yields on these investments to make up for the loss in value as a result.

These yield demands can increase interest rates, which then increases costs for businesses borrowing money to expand. The net effect is a decrease in earnings, which could depress the stock market.

Examples

The U.S. inflation rate by year shows that fluctuations in CPI used to be much worse. Inflation hit 14.4% year-on-year in 1946. The next time it came close to that was 1974, when it hit 11.1% year-on-year while the economy contracted 0.5%. The economic condition where the inflation rate is rising and the economy is contracting is called stagflation.

Deflation occurred between 1930 and 1933. Prices fell 10.7% in September 1932 compared to September 1931. Congress had imposed the Smoot-Hawley Tariff two years earlier, which created a trade war that lowered prices and worsened the Great Depression.

Benefits of the CPI

The CPI measures the rate of inflation, which is one of the greatest threats to a healthy economy. Inflation eats away at your standard of living if your income doesn't keep pace with rising prices—your cost of living increases over time.

A high inflation rate can hurt the economy. Everything costs more, so manufacturers produce less and may be forced to lay off workers. The CPI allows us to gauge these factors.

Key Takeaways

  • The consumer price index measures and reports the effect of inflation and deflation on the economy.
  • The Bureau of Labor Statistics calculates the CPI and publishes percentage changes.
  • The CPI can occasionally give false readings due to variables in the current economy.
  • The Fed uses the CPI to determine whether to modify economic policies to prevent inflation.