Explainer · Inflation · Inflation Day

What Is the CPI? The Monthly Number That Moves Markets (and Sets Our Discounts)

Once a month, at 8:30 in the morning Eastern time, the U.S. government publishes a single percentage. Bond desks reprice trillions of dollars within minutes. Your Social Security check, your tax brackets, and, at Planet Banknote, your discount code all trace back to it. Here is what the Consumer Price Index measures, how to read it, and why banknote collectors watch it with a special kind of appreciation.

How is the CPI calculated?

Government data collectors record tens of thousands of prices every month, in stores, online, and across roughly 50,000 rental housing units, in cities across the country. Each item is weighted by how much of a typical household budget it consumes: shelter carries the heaviest weight by far, with food, transportation, and medical care behind it. Compress all of it into one index, compare it to a year ago, and you get the headline number the newspapers print.

Two versions of that number matter. Headline CPI counts everything. Core CPI strips out food and energy, whose prices swing with weather and oil markets, to show the underlying trend the Federal Reserve actually steers by.

Why does one percentage move so much money?

Because so much is contractually chained to it. Social Security cost-of-living adjustments, federal tax bracket thresholds, inflation-protected Treasury bonds (TIPS), many union wage agreements, and commercial rent escalators all reset off CPI readings. Beyond the contracts, the Fed reads CPI as a report card: a hot print raises the odds of interest rates staying high, a cool print does the opposite, and markets front-run that logic within seconds of release. That is why the release is timed to the second and guarded until 8:30 sharp.

Every note in a hyperinflation collection is a CPI that broke the meter. The full comparison is in our guide to what 3.4% looks like next to real hyperinflation.

A collector holding a Zimbabwe 100 trillion or an Argentine 500,000-austral note is holding what happens when nobody can print the CPI fast enough to keep up with it.

What is Inflation Day at Planet Banknote?

We sell the money inflation destroyed, so we let the CPI set our prices. On the morning each monthly print drops, we take that number and stack it on top of the standing 21% you always get with code PB21, for a limited run of a few days. August's 3.4% print made a 24.4% discount; when the code hit its deadline we killed it, and the next print starts the clock again. Whatever prints, prints: a hot number means a deep discount, a cool number means a modest one. We email subscribers the morning it happens, which is the whole reason to be on the list before the next release lands in mid-September.

Frequently asked questions

When is the next CPI release?

The BLS publishes CPI around the middle of each month, covering the prior month, at 8:30 AM Eastern. The official release calendar is posted at bls.gov, and Planet Banknote emails subscribers the morning each print drops.

Does 3.4% inflation mean everything costs 3.4% more?

No, it is a weighted average. Within one print, shelter might run above 4% while used cars fall outright. The headline tells you what the whole basket did, not what any single shelf did.

What is the difference between CPI and core CPI?

Core CPI removes food and energy prices, which are volatile month to month. Economists watch core for the trend; households feel headline, because nobody gets to skip groceries and gas.

What is the highest inflation the United States ever recorded?

Annual CPI inflation peaked just under 24% in June 1920, and the modern-era high was 14.8% in March 1980. Even those never approached hyperinflation, which economists define as 50% or more in a single month.

Has the U.S. ever had hyperinflation?

Not under the CPI era. The closest American episodes were the Continental currency collapse of the 1770s and Confederate currency in the 1860s, both before modern price statistics existed. For what true hyperinflation does to paper money, see Hungary 1946 and Zimbabwe 2008 to 2026.

Sources

• U.S. Bureau of Labor Statistics, CPI program handbook and release calendar (bls.gov/cpi)
• Federal Reserve statements on inflation targeting
• Cagan, "The Monetary Dynamics of Hyperinflation" (the 50%-per-month definition)