Inflation and CPI: How Prices Are Measured

What Is Inflation?

Inflation is the general rise in the price of goods and services over time. When inflation is positive, each unit of currency buys a little less than it did before — a dollar, euro, or pound stretches less far at the checkout. Economists measure it as a percentage change, almost always compared to a prior period.

Inflation is not just one number; it is an average across hundreds of individual prices, some rising fast, some barely moving, and a few occasionally falling. That averaging process is where the Consumer Price Index comes in.

How CPI Works: The Weighted Shopping Cart

The Consumer Price Index measures how much a representative "basket" of goods and services costs. Statistical agencies — such as the U.S. Bureau of Labor Statistics — survey what typical households actually buy, then assign each category a weight that reflects its share of average spending. Housing costs, for instance, tend to carry a large weight; postage stamps carry almost none.

Think of it as a shopping cart whose contents are fixed by the statisticians. Every month they go back to the same stores and record new prices for the same items. The percentage change in the total bill is the CPI reading.

Headline vs. Core: Why Economists Strip Out Food and Energy

The headline CPI number covers everything in the basket. Core inflation strips out food and energy prices before calculating the rate. This is not because food and energy bills don't matter to real people — they clearly do — but because those two categories are exceptionally volatile and can swing wildly on factors like a drought or an OPEC production cut that have nothing to do with the broader price trend.

Central banks and economists often focus on core CPI to get a cleaner read on the underlying inflation trend. If gasoline spikes one month and then reverses the next, headline inflation will bounce around even if the deeper trend is perfectly stable. Core strips out that noise to reveal the signal. Traders typically watch both numbers when a CPI report lands, because a gap between headline and core can tell very different stories about what is driving prices.

MoM, YoY and the Base Effect

CPI is reported two ways. Month-over-month (MoM) compares this month's basket price to last month's — it is the freshest reading but can be noisy. Year-over-year (YoY) compares to the same month twelve months earlier — it smooths out short-term swings and is the figure most often quoted in headlines. You can learn more about how these percentage-change columns work in our guide to reading percentage moves.

The base effect is a quirk that trips up many readers. Suppose prices surged sharply in one particular month a year ago (the "base" period). Even if prices are rising normally this year, the year-over-year comparison will look artificially low — because you are comparing against an already-high starting point. The reverse is also true: a low base from a year ago can make current inflation look higher than underlying conditions warrant. Economists flag base effects constantly when interpreting YoY data around periods of unusual price movement.

CPI's Cousins: PPI and PCE

The Producer Price Index (PPI) measures prices at the wholesale or factory-gate level — what businesses charge each other before goods reach consumers. Economists read PPI as a pipeline indicator: rising input costs for producers often feed through to consumer prices a few months later, which is why markets watch PPI alongside CPI when assessing the inflation outlook.

The PCE Price Index — Personal Consumption Expenditures — is compiled by the U.S. Bureau of Economic Analysis and is the Federal Reserve's preferred inflation gauge. PCE differs from CPI in two main ways: its basket weights are updated more frequently to reflect how consumers actually shift spending when prices change, and it covers a broader set of goods and services. Core PCE (PCE with food and energy removed) is the specific number the Fed officially targets when setting policy rates. For a deeper look at how those decisions work, see our guide to interest-rate decisions.

Deflation, Disinflation and Stagflation Defined

Deflation is the opposite of inflation: a general fall in the price level, meaning the same basket costs less than it did before. While cheaper prices might sound welcome, sustained deflation is historically associated with serious economic trouble — consumers delay purchases expecting prices to keep falling, which can depress demand and push economies into prolonged slumps. Japan's experience through much of the 1990s and 2000s is the most cited modern example.

Disinflation is often confused with deflation but means something different: prices are still rising, just at a slower rate than before. If inflation runs at 6% one year and then 3% the next, that deceleration is disinflation — not deflation. The price level is still going up, just more slowly.

Stagflation is the painful combination of high inflation and weak or stagnant economic growth — usually accompanied by rising unemployment. It is painful for policymakers because the tools used to fight inflation (raising interest rates) tend to slow growth further, while tools for stimulating growth risk making inflation worse. The 1970s oil shocks produced the most famous stagflation episode in modern economic history.

Why 2% Became the Standard Target

Most major central banks in wealthy economies — including the Federal Reserve, the European Central Bank, and the Bank of England — publicly target inflation of around 2% per year. This number became the de facto standard across the rich world starting in the 1990s, beginning with New Zealand's central bank in 1990 and spreading broadly through the decade that followed.

The reasoning behind 2% rather than zero is that a small positive inflation rate gives central banks room to cut real interest rates in a downturn (it is very hard to push nominal rates below zero), helps prevent the economy from accidentally tipping into deflation, and provides a cushion for measurement error in price indexes. Whether 2% is the right target is a live debate among economists, but the target itself is a widely accepted institutional fact in how central banks communicate today.

Understanding inflation data is foundational to reading almost every other economic release. GDP growth looks very different in real versus nominal terms, as our guide to GDP explains. And inflation interacts with employment, trade, and confidence data across the entire economic indicators landscape. You can track the latest scheduled CPI and PCE releases on the economic calendar.