Educational content. This article explains what YoY means in economic data, how it differs from MoM, and how the two are read in currency markets. It does not constitute investment advice or a trading recommendation. CFD trading carries significant risk of loss and may not be suitable for all investors.
Almost every economic release that moves currency markets is published as two or three percentages at once. US inflation arrives as a monthly change and a twelve-month change; euro area GDP as a quarterly change and a yearly one; US GDP as a quarterly change expressed as an annual rate. Headlines usually quote one of them, economic calendars list them as separate rows, and markets often react to the one the headline did not mention.
This article defines YoY, shows how it is calculated, sets out how it differs from MoM, explains base effects with two real US inflation episodes, covers annualised rates and the different conventions countries use for GDP, and uses the US CPI release of 11 September 2026 to show which number the dollar actually responded to. How inflation itself feeds into the dollar is covered in how CPI affects the US dollar.
What Does YoY Mean?
YoY stands for year-over-year, and it is the percentage change in a figure compared with the same period one year earlier: this August against last August, this quarter against the same quarter last year.
The comparison removes the ordinary seasonal pattern of the data, because it sets a month against the same month rather than against a different season. It appears under several names. Statistical agencies often write "12-month change" or "over the year"; economic calendars write "y/y"; central banks speak of the "annual rate" of inflation. Year-on-year, common in British and European usage, means the same thing. In currency markets the most closely watched YoY figures are consumer price inflation, core inflation, wage growth and, outside the United States, GDP.
How to Calculate YoY
YoY growth is calculated by dividing the change between the current value and the value a year earlier by the value a year earlier, and multiplying by 100.
YoY % = (current value - value one year earlier) / value one year earlier x 100
Inflation is calculated from a price index rather than from prices directly. If a consumer price index stood at 310.0 in August one year and 320.5 in August of the next, the YoY inflation rate is (320.5 - 310.0) / 310.0 x 100, which is about 3.4 percent. The same formula applies to retail sales, wages, industrial output or a company's revenue; only the series changes.
YoY vs MoM: The Two Numbers in Every Release
YoY compares a figure with the same period a year earlier and shows the trend over twelve months, while MoM, month-over-month, compares it with the previous month and shows the most recent momentum.
| Feature | MoM (month-over-month) | YoY (year-over-year) |
|---|---|---|
| Compared with | The previous month | The same month a year earlier |
| What it shows | Latest momentum | Trend over the past twelve months |
| Noise | High; one category can swing it | Low; smoothed across twelve months |
| Seasonal adjustment | Usually seasonally adjusted | Usually not needed, as seasons cancel out |
| Typical size for US CPI | Tenths of a percent | Whole percentages |
| What changes it | This month's prices only | This month entering and a month a year ago leaving |
The US Bureau of Labor Statistics, for example, publishes the monthly CPI change on a seasonally adjusted basis and the twelve-month change on an unadjusted basis, which is why the twelve MoM figures for a year do not add up exactly to the YoY figure. The difference is usually a tenth of a point or two.
The two figures answer different questions. YoY tells how far prices have moved over a year, which is the measure central bank inflation targets are written in. MoM tells whether prices are still rising at that pace now. When the two point in different directions, the market has to decide which one is the news.
Why YoY Can Fall While Prices Keep Rising
YoY inflation can fall while prices are still rising because a YoY rate compares against a year-old base, so it drops whenever the month leaving the twelve-month window was a bigger increase than the month entering it. This is called the base effect.
A YoY rate is, approximately, the sum of the last twelve monthly changes. Each new release adds one month and removes the month from a year earlier. If the month removed was unusually large, YoY falls even if the new month is perfectly ordinary. If the month removed was unusually small or negative, YoY jumps even if nothing unusual is happening now. A falling YoY rate describes slower price growth, which economists call disinflation; it does not mean prices are falling, which would be deflation.
The Base Effect in Numbers
The base effect can be approximated in one line: the new YoY rate is close to the old YoY rate plus the new month's change minus the change from the same month a year earlier.
| Step | Monthly change | YoY rate |
|---|---|---|
| Last month's YoY | 4.0% | |
| Add: this month's MoM | +0.2% | |
| Remove: same month last year's MoM | -1.3% | |
| Approximate new YoY | 2.9% |
Illustrative arithmetic. The approximation ignores compounding and seasonal adjustment.
June 2023: A High Base Drops Out
In June 2023 US CPI inflation fell from 4.0 percent to 3.0 percent YoY, its smallest twelve-month increase since March 2021, while prices rose only 0.2 percent that month, because the 1.3 percent monthly jump of June 2022 left the twelve-month window.
June 2022 was the peak of the post-pandemic inflation surge, with a 9.1 percent YoY rate and one of the largest monthly increases in decades. Exactly one year later that month rolled out of the calculation. Most of the one-point fall in the headline rate was arithmetic from a year earlier, not a change in June 2023's prices, which is the table above with real figures in it.
April 2021: A Low Base Flatters the Number
In April 2021 US CPI rose 4.2 percent YoY, the largest twelve-month increase since September 2008, partly because the base month, April 2020, had seen prices fall during the first pandemic lockdowns.
The monthly increase in April 2021 was genuinely large, 0.8 percent, but it was being compared with a depressed price level a year earlier. For several months in 2021 central banks described the high YoY readings as partly "base effects" for that reason. The episode shows the limit of the argument too: the base effect explained the spring of 2021, but monthly increases stayed high afterwards, and YoY inflation kept rising into 2022.
Annualised Rates: Turning a Monthly Change Into a Yearly Pace
An annualised rate converts a monthly or quarterly change into the yearly rate it would produce if it continued at the same pace, by compounding it over twelve months or four quarters.
Annualised rate = (1 + monthly change)^12 - 1
| Monthly change | Annualised pace |
|---|---|
| 0.1% | 1.2% |
| 0.2% | 2.4% |
| 0.3% | 3.7% |
| 0.4% | 4.9% |
Calculated by compounding each monthly change over twelve months.
This is why a figure that looks tiny can matter. A central bank with a 2 percent inflation target is broadly consistent with monthly readings near 0.2 percent. A 0.3 percent reading, only a tenth higher, is a pace near 3.7 percent a year. Analysts and central banks also watch three-month and six-month annualised rates, which respond faster than YoY without the noise of a single month. The Federal Reserve's 2 percent target itself is defined in terms of the twelve-month change in the PCE price index; the shorter annualised rates are used to judge whether that twelve-month figure is likely to move.
QoQ, Annualised GDP and Why Countries Report Differently
Countries report GDP growth using different conventions: the United States publishes the quarterly change as an annualised rate, while the euro area and the United Kingdom lead with the plain quarter-over-quarter change, and China leads with year-over-year growth.
| Economy | Headline GDP convention | Also published |
|---|---|---|
| United States | Quarterly change at a seasonally adjusted annual rate | YoY |
| Euro area | Quarter-over-quarter change, not annualised | YoY |
| United Kingdom | Quarter-over-quarter change, not annualised | YoY |
| Japan | Quarter-over-quarter change | Annualised rate |
| China | Year-over-year growth | Quarter-over-quarter |
The conventions matter when figures are set side by side. A euro area quarter of 0.5 percent growth compounds to about 2.0 percent annualised, the same pace as a US print of 2.0 percent. Reading the two headlines as 0.5 against 2.0 compares a quarterly number with an annual one. In an economic calendar the suffix, q/q or y/y, and the word "annualised" in the event name are what distinguish them.
Why Forex Reacts to the Surprise, Not to YoY Itself
Currencies react to the difference between an actual figure and the consensus forecast, because the forecast is already reflected in prices before the release, and that surprise can sit in the MoM or core figure while the headline YoY matches expectations.
The US CPI release of 11 September 2026, covering August, shows the pattern in one morning.
| Figure (August 2026) | Actual | Forecast | Previous |
|---|---|---|---|
| CPI YoY | 3.4% | 3.4% | 3.4% |
| CPI MoM | 0.4% | 0.4% | 0.1% |
| Core CPI YoY | 2.4% | 2.4% | 2.5% |
| Core CPI MoM | 0.3% | 0.2% |
Sources: US Bureau of Labor Statistics release of 11 September 2026; forecasts and market reaction as reported by FXStreet.
The headline YoY figure, the one most news reports led with, was exactly as expected. So was the headline MoM figure. Core YoY even edged lower. The only figure that differed from the forecast was core CPI on a monthly basis, one tenth of a point above expectations, which is an annualised pace near 3.7 percent rather than 2.4 percent. The US dollar strengthened against the major currencies on the release, with the dollar index up about 0.15 percent on the day. A reader looking only at the YoY headline would have seen no reason for the move.
This is a description of one release, not a rule that core MoM always leads. Which figure markets weigh most shifts with the inflation environment and with what the central bank has said it is watching. How that weighting feeds into rate expectations and the currency is set out in how CPI affects the US dollar and how FOMC meetings affect the US dollar.
How to Read y/y and m/m in an Economic Calendar
In an economic calendar the same release appears as several rows, each labelled with its comparison period, y/y, m/m or q/q, and each row has its own actual, forecast and previous value.
A US CPI day typically shows at least four rows: CPI m/m, CPI y/y, core CPI m/m and core CPI y/y. The forecast column is the consensus of economists surveyed before the release; the previous column is last month's figure, sometimes revised. Impact ratings are usually assigned per row, and the monthly core figure is often rated as high impact alongside the headline. Wage data in the US jobs report follows the same pattern, with average hourly earnings published as both m/m and y/y, as described in how NFP affects the US dollar. The economic calendar on Vanto lists each release this way, with its time, forecast and previous value.
When YoY Misleads
YoY misleads when the base period was unusual, when a one-off change such as a tax rise or an energy price shock is still inside the twelve-month window, and when a falling YoY rate is read as falling prices.
- Unusual base periods. A lockdown, a price spike or a collapse a year earlier distorts the comparison in both directions, as in April 2021 and June 2023.
- One-off changes. A consumption tax increase lifts YoY inflation for exactly twelve months and then drops out, producing a fall in the rate with no change in underlying pressure.
- Turning points. YoY moves slowly by construction, so it confirms a turn in momentum months after MoM and annualised rates have shown it.
- Level versus change. YoY describes the pace of change. Inflation falling from 4 percent to 3 percent means prices are rising more slowly, not that they are lower.
Frequently Asked Questions
What does YoY mean?
YoY means year-over-year. It is the percentage change in a figure compared with the same period one year earlier, such as August this year against August last year.
What is the difference between YoY and MoM?
YoY compares a figure with the same period a year earlier and shows the twelve-month trend; MoM compares it with the previous month and shows the latest momentum. MoM is noisier and usually seasonally adjusted, while YoY is smoother.
What is a base effect?
A base effect is a change in a YoY rate caused by the figure from a year earlier rather than by the latest period. When an unusually large or small month leaves the twelve-month window, the YoY rate moves even if the new month is ordinary.
How do you annualise a monthly rate?
Add one to the monthly change, raise it to the power of twelve, and subtract one. A 0.3 percent monthly rise, for example, annualises to about 3.7 percent.
Is YoY inflation seasonally adjusted?
Usually not. Comparing a month with the same month a year earlier already removes the regular seasonal pattern. In the US CPI release, the monthly changes are seasonally adjusted and the twelve-month change is not.
Why did the dollar move when CPI YoY matched the forecast?
Because another figure in the same release differed from its forecast. On 11 September 2026 headline CPI YoY matched expectations, but core CPI MoM came in at 0.3 percent against a forecast of 0.2 percent, and the dollar rose.
Put Economic Data Into Context
The economic calendar shows upcoming releases with their y/y, m/m and q/q rows, forecasts and previous values. How inflation, jobs and central bank decisions move the dollar is explained in how CPI affects the US dollar, how NFP affects the US dollar and how FOMC meetings affect the US dollar, and the language central banks use to signal their reaction in hawkish vs dovish. A demo account shows how currency pairs respond to a live release without risking capital.
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