What Is Inflation, and How Does the CPI Actually Work?

Inflation is a ratio, not a feeling

Strip away the commentary and inflation is one fraction: the cost of a fixed set of goods and services now, divided by the cost of the same set earlier, minus one. When the Bureau of Labor Statistics says inflation ran 2.63% in 2025, it means the average price of its basket in 2025 was 2.63% higher than the average price of the same basket in 2024. The basket is the whole trick — everything about the number depends on what goes in it, how it is weighted, and how often the prices are checked.

The headline series is the Consumer Price Index for All Urban Consumers, CPI-U for short. It covers roughly 93% of the U.S. population — everyone living in urban areas, which is where the spending is. It is quoted on a base of 1982–84 = 100, which is why the 2025 annual average reads 321.943: the basket that cost $100 in the mid-1980s costs about $322 now. That base is arbitrary — 100 could have been any number — but the ratios between years are the entire point.

How the basket gets priced

Each month, the BLS collects prices in 75 urban areas from about 6,000 housing units and approximately 23,000 retail and service establishments. The basket holds the things households actually buy: shelter, food, energy, transportation, medical care, apparel, recreation, education, and the long tail of everything else. Each category is weighted by its share of consumer spending, so shelter — the largest single category — moves the headline more than, say, postage stamps.

There is a second number hiding inside every CPI release: core inflation, which strips out food and energy. The September 2026 report is the reason the distinction exists. Headline inflation hit 3.7% year over year that month, but gasoline alone was up 34.2% on the year. Core inflation, at 2.6%, tells you what prices are doing once you remove the two categories that swing the most. The Federal Reserve watches core precisely because it does not want to set interest rates off a gas-price spike.

Seasonally adjusted vs. not, monthly vs. annual

Every CPI release reports two flavors. The not-seasonally-adjusted index is the raw price level — the 321.943 for 2025, the 336.940 for September 2026. The seasonally adjusted percent changes smooth out predictable patterns (heating oil in winter, airfares in summer) so economists can read the trend. When headlines say September prices rose 0.6% on the month, that is the seasonally adjusted figure; the raw index just records the level.

Annual figures work the same way. The BLS publishes an annual average — the mean of the twelve monthly indexes — and the yearly inflation rate is the change in that average. 2025 is the famous exception: federal funding lapsed from October 1 to November 12, 2025, price collection stopped, and no October 2025 index was ever published. The 2025 annual average of 321.943 is the mean of the eleven months that exist. BLS says so explicitly, and the math checks: the eleven published values average to 321.943 exactly.

The formula, in one line

An inflation calculator is the ratio applied to money. Take the index of the later year, divide by the index of the earlier year, and multiply by the amount: $100 × 321.943 ÷ 130.7 = $246.32 turns 1990 dollars into 2025 dollars. There is no model, no forecast, and no opinion in it — just the recorded price history. The average yearly rate is the same ratio raised to one over the number of years, minus one. Between 1990 and 2025 that works out to about 2.61% a year, which compounds into the 146.3% cumulative change.

Three things the index misses

First, substitution. When beef gets expensive and shoppers buy chicken, the fixed basket keeps pricing beef. A chained index (C-CPI-U) accounts for the swap; the headline CPI-U does not, which is one reason economists consider the headline to run slightly hot. Second, quality change. A 2026 laptop at 1990 prices is not the same object, and the BLS adjusts for quality improvements — imperfectly, by everyone's admission. Third, your basket is not the average basket. If your spending is mostly rent and groceries, your personal inflation has run hotter than the headline in recent years, because shelter and food have outpaced the average.

None of this makes the number useless. It makes it a specific instrument: the average price level, measured consistently for over a century, on a public methodology. Use it for what it is — the best available answer to “what would this money have bought then?” — and not for what it is not, which is the cost of your life.

Sources: U.S. Bureau of Labor Statistics — CPI-U series (1982–84 = 100), the September 2026 CPI release (Oct 14, 2026: headline +3.7% YoY, +0.6% MoM; core +2.6% YoY), “Consumer Price Index: 2025 in review” (Jan 21, 2026), and the BLS note that the 2025 annual average (321.943) is the mean of the eleven published monthly values after the October 2025 collection lapse.

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