Researching the RPI, CPI, and CPIH

The ICAEW Library has compiled this guide to help everyone to find the RPI, CPI, and CPIH data they need.

The Library enquiry team can also help ICAEW members, ACA students and other authorised users with requests for consumer price indices data, and related information. Library staff can consult print sources held in our collection on your behalf, and we can loan books and supply documents from our collection by email or post.

If you would like to know more about the information we can offer, please call us on +44 (0)20 7920 8620, email library@icaew.com or contact us through webchat.

The RPI is the most well-known measure of inflation in the UK. Currently calculated by the Office for National Statistics (ONS), it monitors the monthly change in prices of goods and services used by most households. The RPI was preceded by the Cost of Living Index from 1914 to 1947 and the Interim Index of Retail Prices from 1947 to 1956. In an attempt to simplify matters, many publications collectively refer to all of these indices as the RPI.

When looking at historical statistics, note that the RPI moved to a base point of 100 in January 1987. Some listings of the RPI use the original figures but others may have been converted retrospectively to the 1987 base.

There are other indices derived from RPI that have been used for specific purposes, including:

• RPIX, excluding mortgage repayments (formerly used as the base for the UK inflation target);
• RPIY, excluding mortgage repayments and indirect taxes (discontinued in 2017);
• Tax and Prices Index (discontinued in 2017).

In his Pre-Budget Report of 10 December 2003 the Chancellor of the Exchequer announced the UK inflation target would change from one based on RPIX to one based on the Consumer Price Index (CPI).

In 2013, the RPI and its derivatives were found not to meet the required standards for National Statistics; more information on this decision is available from the UK Statistics Authority. The ONS considers the RPI to have a number of shortcomings as a measure of inflation, but it continues to be published due to its use in long-term contracts and index-linked gilts.

The CPI is another important UK measure of inflation. It is used in the government’s target for inflation, and is calculated by the ONS in accordance with European regulations.

It was known as the UK Harmonised Index of Consumer prices (HICP) until 10 December 2003, when the Chancellor announced in his Pre-Budget Report that the UK inflation target would be changed to one based on the CPI. The new inflation target was set at 2 per cent but all other aspects of the framework remained the same; in the UK, the CPI and the HICP are the same index. This was set out in the New Inflation Target letter from the Chancellor to the Governor of the Bank of England.

In addition to the CPI, the ONS also produces a related measure — the Consumer Prices Index including owner occupiers’ housing costs (CPIH). The CPIH is identical to the CPI, with the exception of the inclusion of owner occupiers’ housing costs and Council Tax.

As of 21 March 2017, the CPIH became the ONS's lead measure of inflation.

The Harmonised Index of Consumer Prices (HICP) was developed in 1997 for the purpose of international comparisons. This was a convergence measure required by the Maastricht Treaty in preparation for European monetary union.

Separate HICPs are produced in each EU member state, as well as in several other countries, according to a set of rules developed by Eurostat. HICPs replaced the Interim Indices of Consumer Prices that were published previously.

In the UK, the HICP is now known as the Consumer Price Index (CPI).

The RPI and CPI, as well as the CPIH, are calculated based on the price of 'baskets' of goods and services purchased by a typical UK household.

The contents of the baskets on which the calculations are based differ somewhat between the three indices, and they are revised each year to reflect changes in consumer spending habits.

Full details of the goods and services used to compile the RPI, CPI, and CPIH respectively are published by the ONS on a yearly basis. Broadly, the baskets tend to include items from the following groups:

• Food and non-alcoholic beverages;
• Alcohol and tobacco;
• Clothing and footwear;
• Housing and household services;
• Fuel;
• Furniture and household goods;
• Health;
• Transport;
• Communication;
• Recreation and culture;
• Education;
• Restaurants, catering and hotels;
• Miscellaneous goods and services.

The RPI, CPI, and CPIH all measure the average change from month-to-month in the prices of consumer goods and services purchased in the UK. However, they differ in both coverage and methodology.

The ONS website contains a number of resources which explain what these indices are, what they are used for, and how they are calculated. These include a brief guide to its consumer price indices, a more detailed guide to their respective methodologies, and an article discussing the similarities and differences between them.

Further comparisons between the RPI and the CPI can be found in three older publications, issued in 2011:

• 'History of and differences between the Consumer Prices Index and Retail Prices Index' (ONS)
• 'Implications of the differences between the Consumer Prices Index and Retail Prices Index' (ONS)
• 'The long-run difference between RPI and CPI inflation' (Office for Budget Responsibility)

One key similarity between the CPI, RPI and CPIH is that, within each year, they are all fixed quantity price indices — ie, they measure the change in the price of a fixed basket of items. Changes in price are monitored by comparing the price each month to the price level in the base month of the previous January, producing a series of index numbers for each year.

Furthermore, the CPI, RPI and CPIH all function as annually chain-linked indices: the index numbers for each year are linked together to produce a continuous index series covering several years. This enables the price change for a given year to be compared to that of previous years. When seeking to make comparisons across years, however, one should be aware that the coverage of the indices has changed over time, and that they have been intermittently re-referenced. Further information on the coherence and comparability of the indices is available on the ONS website.

In terms of differences, one important point is that the CPI is based on a common European standard, so the prices it uses differ slightly from those used for the RPI. The indices are also calculated differently. Notably, the RPI aggregates prices at the low end of the spectrum based on the arithmetic mean, whilst the CPI and CPIH predominantly use the geometric mean.

Official RPI data are available back to 1956. In addition to these official data, the ONS also refers to estimates of inflation from 1947 onwards as RPI data. It should be noted that RPI figures have been re-referenced several times, most recently in 1987. There have also been a number of changes to its coverage and to the structure of its sub-components. Information on how the RPI has changed over time can be found in Appendices 1 and 2 in the ONS's Consumer Prices Indices Technical Manual.

The official CPI series runs from 1996 onwards. The methodology and index construction have remained consistent, but the coverage of the CPI has been extended a number of times — details are available on the ONS website. The CPI was re-referenced to 2015 equals 100 on publication of the January 2016 figures.

The CPIH was introduced in 2013, with the series running from 2005 onwards (the earliest that suitable data sources are available). It was re-referenced to 2015 equals 100 on publication of the January 2016 figures.

Indicative modelled estimates for the CPI are available for the period 1950 to 1995, whilst CPIH estimates are available for the period 1950 to 2004.

Current and historical RPI, CPI, and CPIH data are freely available on the Office for National Statistics (ONS) website, via the 'Inflation and price indices' page.

Data from all three indices, along with related information, are also available through various other online and print-based sources.

For detailed information on sources, please see the relevant sections below.

The latest RPI, CPI, and CPIH figures are published by the ONS in its monthly consumer price inflation statistical bulletins. These bulletins include both data and explanatory notes. The most recent bulletin, as well as previous bulletins going back to 2015, can be found on the ONS website.

Datasets and time series for the indices, including the latest data, are also available via the ONS website.

Comprehensive RPI, CPI, and CPIH data — with temporal coverage as detailed in the FAQs section above — are available online via the ONS, in the following time series and reference tables:

• Comprehensive database of time series, covering the RPI, CPI, CPIH, and related measures
• Detailed reference tables for the RPI, CPI, and CPIH (see the contents page of the spreadsheet for coverage details)

Some historical RPI, CPI and CPIH data are also available online in editions of the following ONS publications. Many of these data are the same as those available in the time series database and/or the reference tables, but the presentation of the data may be more useful for quick reference than the detailed ONS reports. When consulting historical sources, beware that a given index may have been re-referenced since the publication of the source in question.

Historical consumer price inflation statistical bulletins:

• Consumer price inflation statistical bulletins: March 2015 onwards
• Consumer price inflation statistical bulletins: January 2010 to February 2015

Historical editions of the Annual Abstract of Statistics (published by the ONS until 2011):

• Annual Abstract of Statistics: 2002 to 2011
  Each edition provides monthly and annual RPI and CPI figures for a number of years preceding the date of publication (plus data for certain other related inflation and price indices, including the RPIX, the RPIY, and the Tax and Price Index).

In addition, in March 2004 the ONS published an article on consumer price inflation between 1750 and 2003. It features a composite price index that allows long-term comparisons of inflation and purchasing power.

The data tables which accompany the annual Department for Work and Pensions publication ‘Abstract of DWP benefit rate statistics’ (formerly known as the ‘Abstract of statistics’) include monthly and annual RPI and CPI data, going back to 1946 in the case of the RPI, and 1988 in the case of the CPI. Recent releases are freely available via GOV.UK.

It should be noted that some of the figures given (pre-1956 in the case of the RPI, and pre-1996 in the case of the CPI) are modelled estimates — see the information given in the FAQs section, above.

The data given in the relevant tables are the same as those available on the ONS website, but the tables may be more useful for quick reference than the detailed ONS reports.

Some UK county councils have maintained webpages containing RPI and CPI data. Again, the data given on these pages are the same as those available on the ONS website, but the tables may be more useful for quick reference than the detailed ONS reports.

For example, the County Treasurer's Department of Hampshire County Council maintained a Retail Prices Index and Consumer Price Index page until 2016. This is no longer updated, but an archived version remains accessible. It shows:

• Monthly RPI figures since 1974;
• Monthly CPI figures since 1988;
• Annual RPI figures since 1962;
• Annual CPI figures since 1988.

In addition, the economy section of the Devon County Council website includes monthly RPI figures (1970-2022), RPI annual percentage change figures (1971-2022); monthly CPI figures (1988-2022), and CPI annual percentage change figures (1989-2022).

It should be noted that pre-1996 CPI figures are modelled estimates — see the information given in the FAQs section, above.

The Bank of England website includes an inflation calculator that can compare the value of the pound between any two years from 1209 onwards. The calculator uses CPI data, both official and estimated.

The most well-known official print source for historical RPI and CPI data is the Annual Abstract of Statistics, which was published by the ONS until 2011. During its existence, it included figures for various inflation and price indices, including the RPI and the CPI. As noted above, some later editions of the Abstract are available online, but many editions are available only in print format. The Library holds print copies of the Abstract for 1886, 1935-1967, 1970-1977 and 1979-2010.

The Library also holds two other official publications covering the RPI:

• Retail prices 1914-1990
• Retail prices indices, 1914-1984

From July 1995 until it ceased being published in 2018, Accountancy magazine included a section giving RPI data for approximately the previous 10 years (though temporal coverage varied somewhat during that period).

Accountancy occasionally also published tables showing the RPI from 1915 to the date of publication, with annual averages for 1915 to 1930 and monthly/annual figures for 1931 onwards. An example can be found in the August 2000 issue.

The Tax Factbook was a looseleaf service from GEE. It gave RPI data in the Checklists section until it ceased publication in 2011.

The historical tax rates and statistics were not included from 2008-2011. Volume 2 of the 2007 edition is retained by the Library and contains the monthly RPI from 1914 to 2008.

Older editions of the FT/ICE Capital Gains Tax Service gave monthly RPI figures from March 1982 onwards, and set out the method for calculating RPI after it was re-based in 1987. It should be noted that the figures were mis-printed in the 1999/2000 edition.

The ICAEW Library collection includes a range of books and articles on the RPI and the CPI.

• View a list of articles and books on the RPI and the CPI

To find out how you can borrow books from the Library, please see our guide to book loans. You can obtain copies of articles or extracts of books and reports by post or email through our document supply service.