How to calculate inflation in the UK: CPI, CPIH and RPI explained
By Dany, RightSums team · Last reviewed · Checked against: ONS, Bank of England, BLS
To work out what money is worth today, multiply the amount by today's price index and divide by the index for the earlier date. According to the ONS, CPI averaged 72.7 in 2000 and was 143.6 in August 2026, so £100 in 2000 is worth £197.52. RPI gives £247.56 because its formula usually shows higher inflation.
Key facts
- UK CPI inflation was 3.1% in the 12 months to August 2026, CPIH 3.3% and RPI 3.4%, according to the ONS (published 16 September 2026).
- £100 in 2000 is worth £197.52 in August 2026 prices on CPI (index 72.7 to 143.6), an average of 2.64% a year.
- UK CPI inflation reached 11.1% in the 12 months to October 2022, the highest in the ONS series since 1989; the annual average for 2022 was 9.1%.
- According to the ONS, the RPI formula adds on average 0.7 percentage points a year compared with CPIH, and the RPI lost its National Statistic status in 2013.
- US CPI-U prices rose 3.4% in the 12 months to August 2026 (BLS index 323.976 to 334.980).
How do you work out what money from the past is worth today?
You work out what old money is worth today by multiplying the amount by today's price index and dividing by the index for the earlier date. A price index is a single number the Office for National Statistics (ONS) publishes each month to track the cost of a fixed basket of goods and services. The ratio between two index numbers tells you how much prices changed between those dates.
The formula is: value today = amount × index today ÷ index then. According to the ONS, the Consumer Prices Index (CPI) averaged 72.7 in 2000 and stood at 143.6 in August 2026, the latest month published on 16 September 2026. So £100 in 2000 is worth £100 × 143.6 ÷ 72.7 = £197.52 in August 2026 prices.
Put the other way round, prices have risen 97.5% since 2000, and £100 kept in cash since then buys 49.4% less. The inflation calculator does this sum for any year or month back to 1800, shows the index numbers it used, and links each one to its ONS or BLS source.
What are CPI, CPIH and RPI?
CPI, CPIH and RPI are the 3 UK price indices the ONS publishes every month, and each one gives a slightly different answer. CPI is the headline measure: according to the Bank of England, the Government has set it a target of keeping CPI inflation at 2%. CPIH is CPI plus owner occupiers' housing costs and Council Tax, which the ONS says make up about 18% of CPIH. RPI is the older Retail Prices Index, which uses a different formula and includes mortgage interest payments.
| Index | What it covers | 12-month rate to August 2026 | Where you meet it |
|---|---|---|---|
| CPI | Household spending on goods and services, excluding owner occupiers' housing costs | 3.1% | The Bank of England's 2% inflation target |
| CPIH | CPI plus owner occupiers' housing costs and Council Tax | 3.3% | The ONS's most complete measure of consumer prices |
| RPI | Older basket and formula, includes mortgage interest and house price depreciation | 3.4% | Older contracts and index-linked gilts |
According to the ONS, CPI rose 3.1%, CPIH 3.3% and RPI 3.4% in the 12 months to August 2026. The next figures, for September 2026, are due on 21 October 2026.
UK inflation rate by year, 2011 to 2025
UK CPI inflation averaged between 0.0% and 9.1% a year from 2011 to 2025, with the peak in 2022. The table shows the annual average rate for each index as published by the ONS (series D7G7, L55O and CZBH). An annual rate compares the average index for the whole year with the average for the year before, so it is lower than the monthly peak: CPI reached 11.1% in the 12 months to October 2022.
| Year | CPI | CPIH | RPI |
|---|---|---|---|
| 2025 | 3.4% | 3.9% | 4.1% |
| 2024 | 2.5% | 3.3% | 3.6% |
| 2023 | 7.3% | 6.8% | 9.7% |
| 2022 | 9.1% | 7.9% | 11.6% |
| 2021 | 2.6% | 2.5% | 4.1% |
| 2020 | 0.9% | 1.0% | 1.5% |
| 2019 | 1.8% | 1.7% | 2.6% |
| 2018 | 2.5% | 2.3% | 3.3% |
| 2017 | 2.7% | 2.6% | 3.6% |
| 2016 | 0.7% | 1.0% | 1.8% |
| 2015 | 0.0% | 0.4% | 1.0% |
| 2014 | 1.5% | 1.5% | 2.4% |
| 2013 | 2.6% | 2.3% | 3.0% |
| 2012 | 2.8% | 2.6% | 3.2% |
| 2011 | 4.5% | 3.8% | 5.2% |
RPI was higher than CPI in every one of these 15 years, by between 0.4 and 2.5 percentage points. The gap compounds: from 2010 to August 2026, £100 grows to £160.63 on CPI but £188.55 on RPI.
Why is RPI higher than CPI?
RPI is usually higher than CPI mainly because of the formula it uses to average price changes. According to the ONS, this "formula effect" has on average added 0.7 percentage points to the annual RPI measure. RPI also uses house prices to estimate housing depreciation, which pushes it up when the property market is strong.
The RPI lost its National Statistic status in 2013, and the ONS says it does not think the RPI is a good measure of inflation and discourages its use. It is still published because some long-running contracts and index-linked gilts are tied to it. The ONS's August 2026 bulletin says that from 2030 CPIH methods and data will be brought into the RPI.
For working out what money is worth, use CPI unless you are checking something that is itself linked to RPI, such as an RPI-linked pension or contract. If you want housing costs counted, use CPIH.
Worked examples with real index numbers
Every example below uses the official index values, so you can repeat the sum with a calculator. A whole year uses the annual average index; a month uses that month's index.
| £100 from | Index | Index then | Index August 2026 | Worth in August 2026 |
|---|---|---|---|---|
| 2000 | CPI | 72.7 | 143.6 | £197.52 |
| 2000 | RPI | 170.3 | 421.6 | £247.56 |
| 2010 | CPI | 89.4 | 143.6 | £160.63 |
| August 2016 | CPI | 100.9 | 143.6 | £142.32 |
| 2020 | CPI | 108.7 | 143.6 | £132.11 |
| 1970 | RPI long run series | 73.1 | 1,663.2 | £2,275.24 |
| 1950 | RPI long run series | 33.0 | 1,663.2 | £5,040.00 |
The 1950 and 1970 rows use the ONS long run RPI series (CDKO), which goes back to 1800, because CPI only starts in 1988. The ONS CPI before January 1997 is itself an estimate, built back from 1988.
Month precision matters over short periods. August 2016 to August 2026 is exactly 10 years at 100.9 and 143.6, a 42.3% rise. Using the 2016 annual average of 100.7 instead would give £142.60, because prices kept rising through 2016.
Has your pay kept up with inflation?
Your pay has kept up with inflation if it rose by more than prices over the same period. To check, multiply your old pay by the price rise and compare the answer with what you earn now. The difference is your real-terms pay change.
Take a salary of £25,000 in 2016 that is £33,000 in August 2026. CPI averaged 100.7 in 2016 and was 143.6 in August 2026, so you need £25,000 × 143.6 ÷ 100.7 = £35,650 to buy what £25,000 bought in 2016. A £33,000 salary is £2,650 short. The cash rise is 32%, but the real-terms change is £33,000 ÷ £35,650 − 1 = a 7.4% cut.
The same sum works the other way: £33,000 today is worth £23,141 in 2016 money. The inflation calculator has a "Has my pay kept up?" mode that does both. These are pay figures before tax. Tax thresholds such as the Personal Allowance that stay the same while pay rises mean more of each rise is taxed, so take-home pay can fall further behind. The pay rise calculator shows a rise after tax.
Total price rise or average a year: which number to use
The total price rise tells you how much more the same shopping costs now; the average annual rate tells you how fast prices rose each year on average. From 2000 to August 2026, CPI rose 97.5% in total. That is an average of 2.64% a year, not 97.5% divided by 26 (3.75%), because each year's rise builds on the one before.
The average annual rate is worked out as (index today ÷ index then) to the power of 1 ÷ years, minus 1. Use it to compare inflation with a savings rate or a pay rise each year. On RPI the same period averages 3.53% a year, which is why an RPI-linked rise is usually worth more than a CPI-linked one.
US inflation: the CPI-U from the Bureau of Labor Statistics
US inflation is measured by the CPI for All Urban Consumers (CPI-U), which the U.S. Bureau of Labor Statistics (BLS) publishes monthly back to January 1913. The CPI-U averaged 172.2 in 2000 and was 334.980 in August 2026, so $100 in 2000 is worth $194.53 in August 2026. Prices rose 3.4% in the 12 months to August 2026, from 323.976 a year earlier.
BLS published no CPI-U index for October 2025, so the series has a gap there. For that month, choose September or November 2025, or the 2025 annual average, which BLS worked out from the other 11 months. The inflation calculator switches between UK and US data with one button.
Common mistakes when adjusting for inflation
The most common mistake is adding up yearly inflation rates instead of multiplying the index ratio. Rates of 9.1% in 2022 and 7.3% in 2023 do not add up to 16.4%: the 2021 to 2023 annual average CPI went from 111.6 to 130.5, a 16.9% rise.
- Mixing indices: dividing an RPI number by a CPI number gives nonsense, because they have different base years (CPI 2015 = 100, RPI January 1987 = 100).
- Using the monthly rate as the annual rate: the ONS headline figure is the change over 12 months, not the change in one month.
- Comparing an annual average with a single month without knowing it: a whole year and a month in the same year give different answers.
- Treating the national figure as your own: CPI is an average household basket. Your costs rise faster if you spend more on things that rose faster, such as rent or energy.
- Using CPI for house prices: house prices are a separate ONS series, and they have risen at a different pace from consumer prices.
Work it out for your own figures
- Inflation Calculator: See what money from any year or month is worth today using official ONS CPI, CPIH and RPI or US BLS CPI-U data, and check if your pay has kept up.
- Pay Rise Calculator: See how much more you take home from a pay rise or raise after tax, and whether it beats inflation. UK 2026/27 and US 2026 rules.
- Compound Interest Calculator: See how savings grow with compound interest. Enter a starting deposit, a monthly top-up, an interest rate and the number of years.
- UK Take-Home Pay Calculator: Work out your UK take-home pay after Income Tax, National Insurance, pension and student loan for 2026/27, per year, month and week.
Frequently asked questions
What is £100 in 2000 worth today?
£100 in 2000 is worth £197.52 in August 2026 prices, using the ONS CPI: 100 × 143.6 ÷ 72.7. On RPI it is £247.56, because RPI usually runs higher. The figure moves every month when the ONS publishes a new index, so use the inflation calculator for the latest month.
Should I use CPI or RPI to adjust for inflation?
Use CPI for most purposes, because it is the UK's headline measure and the one the Bank of England targets at 2%. Use RPI only for things that are themselves linked to RPI, such as an RPI-linked pension or contract. The ONS says RPI is not a good measure of inflation and discourages its use.
What is the difference between CPI and CPIH?
CPIH is CPI plus owner occupiers' housing costs and Council Tax, which the ONS says make up about 18% of CPIH. It is the ONS's most complete measure of consumer prices. In the 12 months to August 2026 CPIH rose 3.3% and CPI 3.1%, according to the ONS.
How is the average annual inflation rate worked out?
The average annual rate is (index at the end ÷ index at the start) to the power of 1 ÷ the number of years, minus 1. From 2000 to August 2026, CPI rose from 72.7 to 143.6 over 26.1 years, which is 2.64% a year. Dividing the total rise by the years overstates it, because prices compound.
How far back does UK inflation data go?
The ONS CPI goes back to January 1988, with figures before 1997 estimated by the ONS. The RPI starts in January 1987 and CPIH in 1988. For older dates the ONS long run RPI series has annual figures from 1800 and monthly figures from June 1947.
When are new UK inflation figures published?
The ONS publishes consumer price inflation once a month, usually on a Wednesday in the middle of the month, for the month before. The August 2026 figures came out on 16 September 2026 and the September 2026 figures are due on 21 October 2026.
How do I check if my pay has kept up with inflation?
Multiply your old pay by today's index divided by the index for the year you were paid it, then compare it with your pay now. £25,000 in 2016 needs to be £35,650 in August 2026 on CPI, so £33,000 is a 7.4% real-terms cut even though it is a 32% rise in cash.
Sources
- ONS: CPI index 00, all items, 2015=100 (D7BT) (retrieved )
- ONS: CPIH index 00, all items, 2015=100 (L522) (retrieved )
- ONS: RPI all items index, January 1987=100 (CHAW) (retrieved )
- ONS: RPI long run series, 1800 onwards (CDKO) (retrieved )
- ONS: CPI annual rate (D7G7) (retrieved )
- ONS: CPIH annual rate (L55O) (retrieved )
- ONS: RPI 12-month rate (CZBH) (retrieved )
- ONS: Consumer price inflation, UK: August 2026 (retrieved )
- ONS: Shortcomings of the Retail Prices Index as a measure of inflation (retrieved )
- Bank of England: Inflation and the 2% target (retrieved )
- BLS: CPI for All Urban Consumers, all items, U.S. city average (CUUR0000SA0) (retrieved )
Terms used in this guide
- Personal Allowance: The Personal Allowance is the income you can earn each UK tax year before Income Tax starts: £12,570 for 2026/27.