Few numbers move markets as reliably as the monthly inflation figure. In the US, that figure is the consumer price index, better known as CPI. On release day, a difference of a tenth of a percentage point from forecasts can swing the dollar, bond yields and share prices within seconds. Understanding what CPI measures, and why traders care so much, is a basic skill for anyone following markets.
What CPI measures
The consumer price index tracks the average change in prices paid by households for a basket of goods and services. In the US it is compiled by the Bureau of Labor Statistics, which collects tens of thousands of prices each month across food, energy, housing, clothing, transport, medical care, education and more. In the UK, the Office for National Statistics publishes its own CPI using a similar approach.
Each category is weighted according to how much households spend on it. Housing costs carry a large weight in the US index, which is why rents and a measure called owners’ equivalent rent have such a big influence on the result. Energy and food carry smaller weights but are more volatile from month to month.
Headline and core
The headline CPI figure includes everything in the basket. Economists also look closely at core CPI, which strips out food and energy because their prices can swing sharply for reasons central banks cannot control, such as weather or conflict. Core is seen as a better guide to the underlying trend. We explain the distinction in more depth in core versus headline CPI for UK traders.
Figures are published in two ways: the change from the previous month and the change from a year earlier. Traders often focus on the monthly core figure because it shows the latest momentum, while the annual figure is what central banks compare with their inflation targets.
Why the monthly figure matters
CPI matters because it shapes expectations for interest rates. The Federal Reserve aims for inflation of 2% over time. Strictly, its preferred measure is the PCE price index, but CPI comes out earlier and feeds into PCE, so it gets more attention from markets.
If CPI comes in hotter than expected, traders may conclude that the Fed will keep rates higher for longer or even raise them. If it comes in cooler, they may bring forward expectations of rate cuts. Because so many assets are priced off interest rate expectations, a surprise in either direction ripples across markets.
How markets react
The typical reaction to a hotter-than-expected CPI figure is higher Treasury yields, a stronger dollar and weaker share prices, particularly in growth stocks whose valuations are sensitive to rates. Gold can come under pressure as the dollar firms and real yields rise. A cooler figure usually produces the reverse.
The size of the move depends on how far the result differs from consensus forecasts and on what was already priced. If markets were braced for a hot figure, an in-line result can trigger relief. The detail matters too: a hot headline driven by petrol prices is treated differently from a hot core reading driven by services.
For UK traders, US CPI often moves sterling against the dollar. Our guide to how interest rate expectations move FX explains the chain from inflation data to currencies.
How to prepare for CPI day
Start by knowing the consensus forecasts for headline and core, both monthly and annual. Note the release time: US CPI is usually published at 8:30am New York time, which is 1:30pm in London for most of the year and 12:30pm during the few weeks each spring and autumn when US and UK clocks change on different dates.
Expect volatility in the minutes around the release. Spreads can widen and prices can jump in both directions before settling. Many experienced traders wait for the first reaction to calm before acting, rather than trying to guess the number.
Look beyond the headline. Check the core figure, the breakdown between goods and services, and housing costs. Watch how two-year Treasury yields respond, as they are closely tied to Fed expectations.
The bottom line
CPI is the market’s monthly check on whether inflation is under control. It moves rates, currencies and shares because it changes what traders expect central banks to do. Learning to study the detail, not just the headline, is what separates a considered response from a knee-jerk one. If you want to test your understanding of how data moves markets, take the free trader assessment.
