Consumer Price Index (CPI)
$$\text{CPI} = \frac{\text{Cost of basket (current)}}{\text{Cost of basket (base year)}} \times 100$$
What is Consumer Price Index (CPI)?
The Consumer Price Index (CPI) is a statistical measure that tracks the average change over time in the prices paid by urban consumers for a representative basket of goods and services. The basket typically includes food, housing, clothing, transportation, healthcare, education, and recreation. National statistics agencies update prices monthly by surveying thousands of retail outlets and service providers.
CPI is expressed as an index number relative to a base period set at 100. The US BLS uses a 1982-84 average as the base for CPI-U; the UK ONS uses 2015=100 for CPIH; Eurostat uses 2015=100 for HICP. The choice of base year affects the magnitude of the numbers but not the inflation rate calculated between any two years.
Consumer Price Index (CPI) explained to a beginner
Think of CPI as the price tag on a very specific shopping cart. Statisticians fill this cart with the same items every month - a loaf of bread, a gallon of petrol, one month of rent, a doctor visit, a pair of jeans. They track exactly how much that fixed cart costs.
When the cart costs 3% more than last year, CPI rose 3% and inflation was 3%. The cart does not change - only its price.
When to use Consumer Price Index (CPI)
Use CPI when converting a historical amount to present-day money (or vice versa). The formula is: Adjusted Value = Original x (CPI later / CPI earlier). CPI is also used to index tax brackets, Social Security benefits, rental contracts, wage agreements, and inflation-protected securities (TIPS). When comparing salaries or prices from different decades, always convert to real (CPI-adjusted) terms first.
Worked examples for Consumer Price Index (CPI)
This table quickly gives you the overview you need to understand Consumer Price Index (CPI) and its most important comparisons.
| CPI measure | Country | Base year | Coverage | Used for |
|---|---|---|---|---|
| CPI-U | United States | 1982-84 | Urban consumers (~93% of population) | Most widely cited US measure |
| CPIH | United Kingdom | 2015 | All households including owner-occupiers | ONS headline measure since 2017 |
| HICP | Eurozone | 2015 | Harmonised across EU countries | ECB policy target |
| CPI | Japan | 2020 | All Japan | BoJ policy reference |
| CPI | Canada | 2002 | Urban households | BoC 2% target reference |
| PCE deflator | United States | 2017 | All personal consumption | Fed preferred inflation measure |
Common pitfalls
CPI uses a fixed basket, which means it cannot fully account for substitution - when prices rise, consumers often switch to cheaper alternatives. This can make CPI overstate true inflation slightly. The PCE deflator (used by the US Fed) uses a flexible basket that adjusts for substitution, which is why PCE typically runs 0.3-0.5 percentage points lower than CPI. Neither measure is "wrong" - they answer slightly different questions.
Frequently asked questions about Consumer Price Index (CPI)
Why are there different CPI measures (CPI-U, CPI-W, CPI-E)?
Different measures use different population baskets. CPI-U covers all urban consumers; CPI-W covers urban wage earners and clerical workers and is used to adjust Social Security; CPI-E covers Americans 62 and over. Older households spend more on healthcare and housing, so CPI-E tends to rise slightly faster than CPI-U, meaning Social Security adjustments may understate actual cost increases for retirees.
How is CPI different from the GDP deflator?
CPI tracks prices paid by consumers for a fixed basket of goods. The GDP deflator tracks the price of everything produced in the economy and adjusts the basket each year to reflect current output. CPI tends to overstate inflation slightly (fixed basket, no substitution); the GDP deflator covers a broader scope. For personal purchasing power calculations, CPI is more relevant.
Does CPI include housing costs?
Yes, but indirectly. The US CPI-U measures housing using "owners equivalent rent" (OER) - what a homeowner would charge to rent their own home. This is a survey-based estimate, not an actual transaction price. OER is the largest single component of US CPI at roughly 26% of the basket. Because OER lags actual home price changes by 12-18 months, CPI can understate housing inflation during rapid price surges.
Quiz: how well do you know Consumer Price Index?
1. What does a CPI basket actually contain?
2. If CPI in 2000 was 172.2 and CPI in 2024 was 314.2, what is the inflation-adjusted equivalent of $500 in 2000?
3. Which US CPI measure does the Federal Reserve use as its primary inflation target?