Every month the Bureau of Labor Statistics publishes one inflation number. Headlines repeat it, and people argue about it over dinner. Behind it sits a set of choices: which goods and services count, how much each one matters, and where the prices come from. Those choices decide what the number can tell you and what it cannot.

A single monthly inflation rate squeezes millions of purchases into one figure. Which items count, and how much each one weighs, shapes what that figure means.
A bright grocery aisle stocked with everyday packaged goods, with a shopper's hand resting on a cart in the foreground, illustrating the everyday items that feed into the inflation basket.
Packaged groceries are one slice of the basket. The index also covers housing, transportation, medical care and much more.

The basket idea

The Consumer Price Index tracks the price of a broad set of goods and services that households buy. Think of it as a very large shopping list. Statisticians do not need to price every product in every store. They need a list that stands in for what people actually spend money on, and a way to follow the prices on that list from month to month.

The BLS explains its design choices in the Handbook of Methods. The chapter on CPI design is the place to start for how items and prices are chosen. Companion chapters cover data sources and concepts. Businesses that get asked to take part in price collection can find their own page for survey participants. A plain-language question and answer page covers the basics.

This article does not restate the sampling rules in detail. Read the handbook chapters for the exact procedures on picking outlets, items and specific products. The details matter, and the agency's own wording is the safest source for them.

Weights decide who moves the needle

Not every price change counts the same. A rise in rent touches far more of a typical household budget than a rise in the price of postage stamps. So the index gives each category a weight based on its share of spending. A heavy category pulls the headline number harder than a light one, even if both change by the same percentage.

The BLS publishes these shares in its relative importance tables. Each row shows a category and how much of the index it represents. It is the quickest way to see whether a surprising monthly change in one category should matter much for the total.

The agency has also posted a comparison of 2026 CPI data using new weights and previous weights. The title makes clear the weights were refreshed. How large the effect was is something to check in the table itself, and this article does not claim a figure. The general point holds either way. Weights reflect a pattern of spending, and spending patterns shift over time.

Core inflation and the food and energy question

You will often hear about core inflation, a version of the index that leaves out food and energy. The usual explanation from analysts is that grocery and fuel prices can swing sharply from month to month, which can hide a steadier trend. That is a common argument, and it is worth checking against how the BLS itself defines the measure before you rely on it.

The trade-off is plain. Nobody can skip eating or driving. A family that sees a jump in gasoline and egg prices will feel it whether or not core inflation holds steady. So the two measures answer different questions. Headline inflation asks what prices did overall. Core inflation asks what the trend looks like once the jumpiest items are set aside.

When the two numbers disagree in the same release, that is not a mistake. It usually means food or energy moved differently from everything else. Look at the category tables to see which items drove the gap.

Checking the index against your own budget

The CPI measures price changes for a broad group of consumers. It does not measure your cost of living. A homeowner with a fixed mortgage who cooks at home and takes the bus faces a different mix than a renter who drives to work and pays for child care. The headline blends many such households.

You can test how well the national figure fits you with a short exercise.

  1. Pull the last three months of bank and card statements and sort spending into broad groups such as housing, food, transportation, and medical care.
  2. Work out what share of your total each group takes.
  3. Open the BLS relative importance table and compare those shares with the national ones.
  4. Look at the monthly changes for the categories where your share is much larger than the national share.

If you spend far more on rent than the national share, the shelter figures will tell you more about your year than the headline will. If you drive little, a fuel spike matters less to you than the news suggests. The point is not to replace the official number. It is to read it with your own receipts beside it.