Monthly Consumer Price Index index using 2015 as a base with the index to 3 decimal places

​FOI Reference: FOI/2022/4266

You asked

I am requesting tables for:

Consumer Price Index (CPI) showing the all items monthly index and annual percentage change using as a base 2015 as 100 from 1988 to April 2022 and on the same basis historical data back to 1947, with the index to 3 decimal places if possible.

We said

​Thank you for your query requesting the CPI all items monthly index and annual percentage change from 1947 to April 2022 using 2015 as a base, with the index to 3 decimal places.

Unfortunately, we do not hold all the information you require. For example, we do not hold index numbers for 1947 to 1948, nor annual percentage changes for 1947 to 1949. Please see accompanying download which contains a spreadsheet of available data. This has been extracted from various sources on our website.

Column C of the spreadsheet contains indices for January 1988 to date which are referenced on 2015=100. These indices have been taken from table 57 of our monthly published reference tables.

Column B contains indices for January 1949 to December 1988 but these are referenced on 1965=100 and are to one decimal place only. This information is available from table 6 of a modelled historical dataset released on 18 May 2022. As was stated in an accompanying article, these historical estimates are purely indicative and are provided for analytical purposes only. Care should be taken not to over-interpret the accuracy of the data given the uncertainty involved in extrapolating the series so far back in time. Given this, the figures have only been published to one decimal place. More information on the way in which the historical series was produced is available from Consumer price inflation, historical estimates, UK, 1950 to 1988 - methodology.

The historical series was not published on 2015=100 because it would result in indices for the earliest years that are particularly small (around 4.5) and would suffer from rounding issues that can arise from small index values. It is possible to calculate an index on 2015=100 for pre-1988 by taking a ratio of the January 1988 index referenced on 2015=100 to the January 1988 index referenced on 1965=100, and applying this ratio as a factor to earlier indices. Additional care needs to be taken with the resulting indices because of the degree of rounding in addition to the uncertainties with the modelling.

The annual percentage change is included in column D of the spreadsheet. The figures for January 1950 to December 1988 are taken from the historical dataset linked above. Although based on indices referenced on 1965=100, the percentages are consistent with ones which would be calculated if the indices were referenced on 2015=100. The percentage changes for subsequent periods from January 1989 are available in the time series part of our website.