The simple version

When a headline says inflation is running at some percent a year, it is often taking one month's change and stretching it across twelve. That is called annualizing.

An annualized rate is a projection of one month, not a measurement of a year. Today's report shows why the difference matters.

The numbers

• The personal consumption expenditures (PCE) price index rose 0.3% from July to August 2026 (Bureau of Economic Analysis, Personal Income and Outlays, August 2026, released September 30, 2026)
• The same index rose 3.4% from August 2025 to August 2026 (Bureau of Economic Analysis)
• Excluding food and energy, the index rose 0.2% for the month and 3.0% from a year earlier (Bureau of Economic Analysis)
• The index rose 0.1% in July, as shown in today's release (Bureau of Economic Analysis)
• The Federal Reserve's 2% inflation goal is measured by the annual change in the PCE price index (Federal Reserve Board)
• 0.3% a month, compounded over 12 months, is about 3.7% (our arithmetic)

What annualizing does

Annualizing asks a what-if question: what would happen if this month's pace held for a full year? The answer is a yearly pace, not a yearly result.

The measured 12-month change answers a different question, which is what actually happened over the past year. We covered why a monthly and a 12-month change can point opposite ways in our piece on used car prices.

Why you cannot just multiply by 12

Prices that rise every month rise on top of the previous month's increase, the same way interest compounds. So 0.3% a month becomes about 3.7% over a year, not 3.6%.

Multiplying by 12 gets close when the monthly number is small. The gap grows as the monthly number gets bigger.

The Real Cost lens on a rounded number

The report gives monthly changes to one decimal place. Annualizing a rounded number stretches the rounding too.

• A monthly change shown as 0.3% could be anything from about 0.25% to about 0.35% before rounding.
• Annualized, 0.25% a month is about 3.0% a year, and 0.35% a month is about 4.3% a year.
• So one printed 0.3% fits a yearly pace anywhere from about 3.0% to about 4.3%.
• July's 0.1% annualizes to about 1.2%. Two neighboring months of the same index point to yearly paces about 2.5 points apart.
• A rounding difference of a twentieth of a point in the monthly figure becomes more than half a point once it is annualized.

The 12-month rate is rounded too, but nothing stretches it. Its 3.4% is off by no more than a twentieth of a point either way.

What this means

An annualized monthly figure can be useful for spotting a change in direction early. It can also swing hard on rounding and on one unusual month.

The Federal Reserve writes its goal in terms of the annual change. When a headline gives a yearly pace built from one month, it helps to know which of the three numbers you are looking at.

What this is NOT

This article explains how inflation rates are expressed. It does not forecast inflation, interest rates, or Federal Reserve decisions, takes no position on monetary policy, and gives no investment or financial advice. The annualized figures and rounding ranges are our arithmetic from the monthly changes the Bureau of Economic Analysis published, not figures the bureau published. Today's release includes an annual update that revised earlier months back to January 2021, so July and earlier figures may differ from what was reported at the time.

Sources

• Bureau of Economic Analysis, Personal Income and Outlays, August 2026, released September 30, 2026: https://www.bea.gov/news/2026/personal-income-and-outlays-august-2026
• Federal Reserve Board, Why does the Federal Reserve aim for inflation of 2 percent over the longer run?: https://www.federalreserve.gov/faqs/economy_14400.htm