There is no single answer, because there is no single index
One hundred pounds from 1988 is £279 today on CPI. It is £377 on RPI. Both figures come from the same statistical agency, both are published monthly, both are correct on their own terms, and they are 35% apart.
That is not a rounding difference or a question of which vintage of the data you happened to download. RPI is calculated differently from CPI and runs persistently above it, by around a percentage point a year. Across a single year nobody would notice. Across a working lifetime it compounds into a third of the answer.
So a calculator that prints one number has already made the decision that matters and has not mentioned it. This one prints both, every time, side by side, and shows the gap in pounds.
The clearest illustration is 2009
In 2009 the RPI index fell while the CPI index rose. Prices in the economy did not do two things at once: RPI includes mortgage interest payments, interest rates collapsed that year, and that pulled the whole index down while other prices carried on rising.
Whether that belongs in a measure of the cost of living is a real argument with real answers on both sides. What it is not is a technicality, because if you were arguing about a pay settlement or a rent review in 2010, which index the agreement named decided the outcome.
Which index applies to you is a question about your paperwork
Different things in the UK are uprated by different indices. Where a contract, a pension scheme, a lease or a loan agreement names one, it applies to you, whatever any calculator says and whatever the more favourable figure happens to be.
So the useful thing this page can tell you is the size of the difference, so you know it is worth going and reading the wording. On a long-running agreement the choice of index is often worth more than anything else you could negotiate in it.
Inflation compounds, so the total is not the rate times the years
This is the arithmetic error people make when they try to sanity-check a figure. Total change divided by the number of years gives a bigger number than the rate that compounds to it, and the longer the span the wider the gap.
Each year's inflation applies to the previous year's prices, not to the original ones. The tool shows the cumulative change, the compounding rate, and the naive average alongside, so you can see how far apart the last two are before quoting either.
Read it backwards for a wage, not forwards
Forwards answers "what would that old price be now", which is an interesting question. Backwards answers "what is my current salary worth in the money I used to earn", which is usually the question somebody has.
The tool gives both directions from the same two years. If you are working out whether a series of pay rises kept pace, the backwards figure is the one to use, and comparing it against what you were paid then is a more honest test than any percentage.
Where these numbers stop, and why
The tables here are ONS series D7BT for CPI and CHAW for RPI, read directly from the ONS on 15 August 2026, and checked against published annual rates before use.
CPI as published begins in 1988 and RPI in 1987. Figures for earlier years exist, but they are modelled historical estimates rather than the index itself. This tool refuses those years instead of extrapolating into them, because presenting a model as a measurement is precisely the failure it exists to avoid.
They are also annual averages, not a point in time, which is a deliberate choice about staying current. A question of the form "what is money from one year worth in another" needs one figure per year, so this goes out of date once a year rather than every month, and a year that has not finished has no annual average and is simply not offered.
Why the official figure never matches what it feels like
An index is an average across a basket of goods, and nobody buys the basket. It is a weighted picture of everybody's spending at once, which means it describes almost nobody exactly.
If most of your money goes on rent, energy and food, your own inflation over the last few years has been well above the headline rate, because those are precisely the categories that moved most. If you own your home outright, drive very little and eat at home, it has been below. Neither of those makes the published figure wrong. It describes the economy, not your household, and it was never meant to do anything else.
Common questions
What is £100 from 1988 worth today?
There are two official answers and they are 35% apart. On CPI it is £279. On RPI it is £377. Both indices are published by the ONS, both are correct on their own terms, and any calculator that gives you a single number has quietly made that choice on your behalf.
Why do CPI and RPI give different answers?
They are calculated differently, and RPI runs persistently above CPI by roughly a percentage point a year. Over one year that is barely noticeable. Over thirty years it compounds into about a third of the answer. It is also why 2009 shows prices falling on RPI while rising on CPI: RPI includes mortgage interest and rates collapsed that year.
Which index should I use?
Whichever one governs your particular number, and that is worth reading rather than assuming. Different things in the UK are uprated by different indices, and where a contract, a pension scheme or a loan agreement specifies one, that index applies to you whatever a calculator says. Check the wording before using either figure in an argument with anyone.
Why can I not go back before 1988?
Because CPI as published begins in 1988 and RPI in 1987. Figures for earlier years do exist, but they are modelled historical estimates rather than the index itself. This tool refuses those years rather than extrapolating into them, because presenting a model as a measurement is exactly the thing it is trying to avoid.
Why is this year not in the list?
These are annual averages, and a year that has not finished does not have one. The tables run to the last complete year and will pick up the next one when the ONS finalises it. That is a deliberate choice: a year-to-year purchasing power question needs one figure per year, so the tool goes stale once a year rather than every month.
Is inflation over 25 years just the annual rate times 25?
No, and the shortcut always overstates the rate. Inflation compounds: each year applies to the previous year prices. Dividing the total change by the number of years gives a bigger figure than the rate that compounds to it. The tool shows both so the gap is visible.
Why does the official figure not match what I feel?
Because an index is an average across a basket and nobody buys the basket. If most of your money goes on rent, energy and food then your own inflation over recent years has been well above the headline. If you own your home outright and drive very little it has been below it. The figure describes the economy, not your household.