A flat grocery bill can coexist with rising inflation because the Consumer Price Index tracks more than food. It combines price changes across a broad basket of household purchases, with categories weighted by their importance in consumer spending. In September 2026, the food index did not move, but gasoline and shelter did. Their increases, along with gains in other categories, pushed the all-items measure higher. The apparent contradiction is a matter of scope: one category can be steady while the overall basket becomes more expensive.
The total is a weighted mix
The CPI is an average change in prices paid for a representative basket of goods and services. The Bureau of Labor Statistics groups purchases into categories and weights their changes according to their importance in the spending patterns of the population the index represents Bureau of Labor Statistics. Food is only one part of that basket. Housing, transportation, medical care, apparel, recreation and other items also count.
That weighting matters. A large price move in a category with substantial weight can influence the total more than a similar move in a smaller category. The all-items figure is not a simple tally of how many prices rose or fell, nor does a flat food index cancel every increase elsewhere. It summarizes the combined movement of the basket.
September’s numbers show the arithmetic
In September, the seasonally adjusted all-items CPI rose 0.6% from August. The food index was unchanged, while gasoline rose 9.1% and shelter rose 0.4%. The energy index increased 5.2%, and gasoline accounted for more than half of the monthly all-items increase, according to the September release Bureau of Labor Statistics.
Even within food, a flat overall reading concealed different directions: food at home fell 0.1%, while food away from home rose 0.2%. A household that mostly buys groceries may see a different pattern than one that often eats out. And a steady food index says nothing about the bill for driving or housing.
One month and one year tell different stories
The monthly change and the year-over-year change answer separate questions. The September release reported that the all-items index was 3.7% higher than a year earlier, while food was up 2.5% over that period. Gasoline was up 34.2% year over year, and shelter was up 3.2% Bureau of Labor Statistics. A flat reading for food in September does not mean food prices were flat over the year.
A monthly index measures movement during that month; it does not say that prices returned to an earlier level. If an item rose in previous months and then held steady, its index remains elevated even without a new monthly increase. Likewise, a year-over-year rate reflects the cumulative change since the same month a year earlier.
A national average is not a household budget
The CPI-U represents the spending patterns of urban consumers, not a specific family. People differ in what they buy and how much of their budget goes to each category. BLS notes that someone with unusually high medical expenses, for example, can experience a different rate of price change from the published average Bureau of Labor Statistics. The same principle applies to households especially exposed to rent or fuel costs.
So when grocery prices seem steady but inflation rises, both observations can be accurate. The grocery index describes one slice of the basket; the all-items index reflects the weighted movement of the whole. The September data make that distinction visible: unchanged food alongside higher gasoline and shelter, and a higher overall measure Bureau of Labor Statistics.
Sources & further reading
- September 2026 Consumer Price Index ↗Bureau of Labor Statistics
- Consumer Price Index questions and answers ↗Bureau of Labor Statistics
