12-month rate
The change in the price index versus the same month a year earlier — the number you see in every headline.
In plain words
The 12-month rate — also called year-on-year (YoY) — asks a single question: how much more does the same basket of goods cost today compared to exactly twelve months ago?
If the CPI was 120.0 in June last year and is 123.5 this June, the 12-month rate is (123.5 ÷ 120.0) − 1 = +2.9%. That's the number in the headline. The price level rose 2.9% over the past year.
It's the world's most quoted inflation figure because it's intuitive (one year is easy to relate to), timely (updated every month), and automatically seasonal — comparing the same month strips out predictable swings like Christmas shopping or summer fuel.
How it's calculated
π(t) = I(t) ÷ I(t − 12) − 1 The 12-month (year-on-year) inflation rate I(t) CPI index value this month I(t − 12) CPI index value exactly 12 months ago (same calendar month) Why "same month" matters
Comparing to the same calendar month automatically strips out seasonal price swings — no adjustment needed.
Retail prices spike every December due to gift demand. Comparing Dec to Dec means both periods have the same Christmas bump — it cancels out.
Petrol prices rise every summer as holiday driving increases. July vs July last year: both summers, same seasonal lift — it cancels out.
Only genuine price-level changes that differ from a year ago. A one-off energy shock, a tariff change, or a supply disruption — things that are new relative to last year.
12-month rate vs annual average
Both measure inflation, but they answer different questions — and can give very different numbers in a fast-moving year.
Watch out: the base effect
The 12-month rate depends heavily on what prices were doing a year ago — the "base." A high base drags the rate down even if nothing new happened.
Common misreadings
"The 12-month rate fell, so inflation is over."
A falling 12-month rate means disinflation — prices are rising more slowly. It could also reflect a base effect. Neither means prices are falling or that the inflation episode is resolved.
"The monthly number is what really matters."
Month-to-month changes are noisy — a single month can be distorted by one-off events. The 12-month rate smooths this by spanning a full year. Both matter, but 12-month is more comparable across time.
"8% inflation last year means I need 8% more money."
The 8% is an average across all goods. Your personal inflation rate depends on your spending basket — if you spend heavily on energy or food, you likely felt more than 8%. If you own your home and rarely travel, perhaps less.
Frequently asked questions
What is a 12-month inflation rate?
It compares the price level this month to the same month exactly twelve months earlier, expressed as a percentage change. It is the most widely reported inflation figure in the media — the number that appears in news headlines every month.
Is the 12-month rate the same as the annual average?
No. The 12-month rate is a point-in-time reading comparing one month to twelve months prior. The annual average smooths all twelve monthly readings in a calendar year. They often differ significantly, especially when inflation is rapidly changing — as in 2022 when monthly readings ranged from 6.5% to 9.1% but the annual average was 8.0%.
Why do different countries report different inflation figures for the same month?
Different countries use different price indexes (CPI vs HICP vs PCE), different basket compositions, different weighting methodologies, and different base years. The 12-month rate formula is the same, but what goes into I(t) varies. This is why you can't directly compare a US CPI figure to a eurozone HICP figure without adjusting.