How the calculator works
Inflation is the rate at which prices rise over time, and the Consumer Prices Index (CPI) is the UK's headline way of measuring it. Every month the Office for National Statistics prices a huge basket of goods and services, from bread and bus fares to streaming subscriptions, and turns the result into an index. This calculator uses the official CPI all items index as annual averages (2015 = 100), covering 1988, when the series begins, to 2025, the latest complete year.
So if the index doubled between your two years, prices on average doubled, and you would need twice as much money to buy the same basket. The calculation works in both directions: run it backwards to see what today's money was worth in an earlier year.
The second mode ignores the historical data and simply compounds an average rate you choose. It is useful for questions the CPI series cannot answer, such as "if inflation averages 2% for the next 15 years, what will £10,000 of today's spending cost then?", or for testing how sensitive a plan is to 2% versus 4%. Small differences in the rate compound into large differences over decades, which is exactly why inflation matters.
Worked examples
CPI, CPIH and RPI: which measure?
You will see three inflation measures in UK news. CPI is the headline measure, used for the Bank of England's 2% target and for uprating many benefits. CPIH is CPI plus owner-occupiers' housing costs, and is the ONS's preferred measure. RPI is an older index with a different formula that usually runs higher; it lost its national statistic status in 2013 but still lingers in some contracts, student loan interest and rail fare rises. This calculator uses CPI because it is the standard answer to "what has inflation been?" and has a consistent official series back to 1988.
What the result can and cannot tell you
CPI is a national average, so your personal inflation depends on what you actually buy. If your spending leans heavily on energy, rent or food, your costs may have risen faster than the headline number; if it leans on electronics, probably slower. The calculator also says nothing about wages or investment returns, only about prices. To check whether savings kept up, compare the result with what the same money would have earned in an account using the compound interest calculator.
Figures are for guidance, not financial advice. The underlying data is published free by the Office for National Statistics.
Frequently asked questions
Where does the inflation data come from?
From the Office for National Statistics. The calculator uses the CPI all items index (series D7BT, 2015 = 100) as annual averages, covering 1988 to 2025, the latest complete year. The figures were downloaded from ons.gov.uk and cross-checked against a second published source before being included.
What is the difference between CPI and RPI?
CPI (the Consumer Prices Index) is the UK's headline inflation measure and the one used here. RPI (the Retail Prices Index) is an older measure with a different formula and coverage that usually produces higher figures. RPI lost its national statistic status in 2013, though it still appears in some contracts and rail fare rises.
Why do the results only go back to 1988?
The official CPI index starts in 1988. The ONS publishes modelled estimates for earlier years, but they are not part of the official series, so this calculator sticks to the confirmed data. For a longer historical view, the Bank of England publishes its own inflation calculator going back centuries.
Can I estimate future inflation?
Not from the CPI data, because nobody knows future inflation rates. Use the custom rate mode instead: pick an assumed average rate, say 2% (the Bank of England's target), and a number of years, and the calculator compounds it for you. Treat the result as a what-if, not a forecast.
Why does the calculator use annual averages rather than monthly figures?
Because the question people actually ask is what an amount from a given year is worth now. Annual averages smooth out month-to-month noise and match how the ONS reports calendar-year inflation. If you need month-level precision, the full monthly index is available free on the ONS website.