The basic idea
CPI answers one question: how much more (or less) does it cost to buy the same basket of consumer goods and services than it did before? The Bureau of Labor Statistics collects tens of thousands of price quotes each month — groceries, rent, gasoline, haircuts, insurance, phone plans — across dozens of urban areas, weights them by how much a typical household actually spends on each category, and rolls the result into a single index number. The percentage change in that index is the inflation rate. This site's inflation calculator runs on the headline version of this index, CPI-U, which covers roughly 93% of the U.S. population.
The weights
Weights come from the Consumer Expenditure Surveys, in which households report what they spend, and since 2023 the BLS has updated them annually. Housing dominates: shelter is around a third of the index. The largest single component, owner's equivalent rent, is also the most debated — homeowners are asked what their home would rent for, because CPI aims to measure the cost of living in the house, not the investment value of owning it. This is why CPI can look calm while home prices boom: house prices are an asset price, and CPI deliberately excludes asset prices.
Headline vs core
"Headline" CPI includes everything. "Core" CPI strips out food and energy — not because they don't matter, but because they swing with weather and geopolitics, and policymakers want to see the underlying trend. The Fed's official 2% target is actually defined on a different index altogether, the PCE price index, which weights categories by broader spending data and typically runs a few tenths below CPI. When the Fed and the headlines seem to disagree about inflation, differing indexes are often why.
Adjustments and criticisms
Two methodological choices draw the most criticism. Substitution adjustment lets the basket shift when consumers switch from beef to chicken as relative prices change, which critics argue bakes in a declining standard of living. Hedonic adjustment strips out price increases attributable to quality improvements — if this year's laptop costs the same but is twice as fast, hedonics record a price decline. Defenders note both adjustments cut in either direction and aim to keep the basket comparable over time; critics respond that the cumulative effect flatters the official number. Both things can be partly true, which is a reason to check price-based measures against quantity-based ones like money supply growth — the comparison the money supply calculator is built around.
What CPI is not
CPI is not a cost-of-living guarantee for any individual household — your basket differs from the average. It is not a measure of asset inflation: stocks, houses, and bonds are outside it entirely. And it is not the only lens on the dollar's value. It is, however, consistently measured, published monthly since 1913, and the basis for Social Security adjustments, tax bracket indexing, and TIPS. Understanding what's in it is prerequisite to arguing about it.
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