Fundamental analysis · Beginner

Inflation and the Consumer Price Index

The annual inflation rate can fall 0.7 points in a month when current prices barely moved, because the month it is compared against dropped out.

Last reviewed 15 September 2026 by the MarketPro research desk · Editorial policy

In short

The Consumer Price Index measures the change in price of a fixed basket of goods and services, reported as a monthly and an annual rate. Core CPI strips out food and energy to show the underlying trend. It matters to traders because it is the main input into interest rate decisions, and a surprise against consensus reprices rate expectations within seconds, moving currencies and bonds first.

What to remember

A 0.3% monthly core print annualises to 3.7%. Anything at or above that is inconsistent with a 2% target held over time.
Base effects move the annual rate when current prices have not changed, because a large old month drops out of the comparison window.
Core and services drive the rate path. Headline drives household behaviour. They are different questions with different answers.
Spreads on majors can widen from under a pip to several pips in the first minute after the print, so a stop inside that range fills where the market chooses.

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Inflation is the rate at which prices rise, and the Consumer Price Index is the most closely watched way of measuring it. CPI prices a fixed basket of goods and services, compares it to the same basket in an earlier period, and reports the change. It matters to traders for one reason above all others: it is the input that most directly determines what a central bank does next.

A 0.4% month that reads as 4.9% a year

CPI is published as both a monthly and an annual change, and the two can tell opposite stories. Suppose the monthly print is 0.4%. Compounded across twelve months that is an annualised pace of 4.9%, because 1.004 raised to the twelfth power is 1.049. If the year on year figure that day is 3.1%, the headline says inflation is falling and the monthly detail says the current run rate is well above it.

Which one is right depends on what you are trying to know. The annual figure tells you where prices are relative to a year ago. The monthly figure, annualised, tells you the pace right now. Central bankers watch three-month and six-month annualised rates precisely because those cut through the base effect problem below, and because policy has to respond to the current run rate rather than to last year's.

A practical rule: any monthly core print at or above 0.3% is inconsistent with a 2% target held over time, because 0.3% monthly annualises to 3.7%. Two or three of those in a row is what shifts rate expectations, not a single one.

Base effects: the number moves when nothing does

The year on year rate is a comparison between two numbers, and the older one changes every month. Suppose prices rose 0.9% in the same month last year and 0.2% this month. The annual rate falls by roughly 0.7 percentage points on that alone. Nothing about current prices changed. A large old month simply dropped out of the window and a small new one replaced it.

This is why forecasters publish a path for headline inflation months in advance and are often close. Most of the next few prints is already determined by what is rolling off. When commentary says inflation is "expected to fall to 2.6% by March", a good part of that is arithmetic rather than prediction, and traders who treat the eventual decline as a surprise are reacting to something that was visible in advance.

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Headline, core, and the part that is just petrol

Headline CPI includes everything, food and energy included. It is what households actually experience and what inflation-linked contracts often reference.
Core CPI strips out food and energy. Not because those do not matter, but because they are volatile enough to obscure the underlying trend that policy can influence.
Services and shelter are the components central banks watch hardest, because they are sticky. A goods price falls when a shipping bottleneck clears. A wage-driven services price does not.
PPI measures costs at the wholesale level and lands before CPI in most calendars, which is why it sometimes shifts the consensus for the consumer print a few days later.

An energy spike can push headline well above core for months and then reverse. If you are trading the rate path, core and services are the numbers that move it. If you are thinking about consumer demand, headline is the one households respond to. The two questions have different answers and the release gives you both.

Ninety seconds after the print

A significant CPI surprise is one of the fastest repricing events on the calendar. Rate futures move first, then the currency, then equities. Spreads on major pairs can widen from under a pip to several pips for the first minute, and a stop placed inside that range is liable to be filled at a price you did not choose. A market order at that moment guarantees execution but not price.

The practical consequences are unglamorous. Know the release time in your own timezone. Decide before the number whether you are holding, flat or reduced, and decide it on size rather than on hope. If you do hold through it, calculate what the plausible range costs you first, which is what the pip calculator and a realistic stop distance are for. Event risk is one of the few risks you can schedule.

Reading the release rather than the headline

Open the release and check four things in order: core month on month, the services component, whether the previous month was revised, and how the print compares to the consensus that was live that morning rather than the one published last week. That takes about two minutes and it is where the actual information is.

One release rarely changes policy. Committees weigh a trend across several reports and they say so explicitly in their statements. What one release does change is the market's probability distribution for the next meeting, and that distribution is what currency pricing responds to, not the inflation rate itself.

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Written and reviewed by the MarketPro research desk

MarketPro is a trading-signal and trading-education app operated by Harajuku Holdings LTD (Cyprus). Our signal desk publishes and tracks every trade idea in-app, and every page here is checked against the app's live behaviour before publishing. Last reviewed 15 September 2026. How we produce signals and content · About MarketPro

FAQ

Inflation and the Consumer Price Index: questions

Why do central banks target core inflation instead of headline?

Food and energy prices are set largely by global supply, weather and geopolitics, none of which respond to interest rates. Including them adds volatility that policy cannot influence and that tends to reverse. Core inflation, and especially the services component, reflects domestic wage and demand pressure, which is what monetary policy can actually act on. Headline is still the official target in most mandates, but core guides the decision.

What is the difference between CPI and PPI?

CPI measures prices paid by households for a basket of consumer goods and services. PPI measures prices received by producers at the wholesale level, before goods reach the shop. PPI usually lands earlier in the calendar and is sometimes treated as a leading indicator of consumer prices, though the pass-through is incomplete and can take months, so a PPI surprise does not reliably predict the CPI print.

How much does a CPI surprise move a currency?

It depends on how far the print is from consensus and on how much of the next policy move is already priced. A deviation of 0.1 percentage point on core is often absorbed quickly. A 0.3 point surprise in either direction can move a major pair by 60 to 100 pips within minutes because it forces a repricing of the expected rate path, which is the actual driver rather than the inflation rate itself.

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