National population projections, background and methodology: 2018-based

We publish national population projections by age and sex for the UK and its constituent countries every two years. We base them on the latest mid-year population estimates together with assumptions of future levels of fertility, mortality and migration. The primary purpose of the projections is to provide information on potential future population levels. They are used as a common framework for national planning in a number of different fields.

The Government Actuary’s Department (GAD) produced the first projections of the population of the UK in the 1920s. These earliest projections were mainly used in connection with long-term financial estimates under the Contributory Pensions Acts and other schemes of social insurance. However, projections made since the War have been increasingly used in all areas of government planning. The GAD produced projections each year from 1955 to 1979 and then every second year until 1991. There was then a 1992-based set, and since then projections have reverted to being produced every second year. The Office for National Statistics (ONS) took over responsibility for the production of the national population projections in 2006.

We occasionally produce additional “interim” projections. We published 2001-based projections following the 2001 Census and an additional set based on the 2003 estimates to incorporate revisions to the population estimates for England and Wales.

The main focus of the 2018-based projections is on the next 25 years up to 2043, though we also produce longer-term projections to 2118. The uncertainty of population projections increases the further they are carried forward and particularly so for smaller geographical areas and age–sex breakdowns. In addition to the principal projections, we also make available variant projections, based on alternative assumptions of future fertility, mortality and migration. For more information on how ONS projections meet user needs along with information on their fitness for purpose, please see the Quality and Methodology Information (QMI) report.

The 2018-based projections supersede the 2016-based projections published on 26 October 2017.

We produce the projections on behalf of the National Statistician and the Registrars General for Scotland and Northern Ireland. We agree the underlying assumptions in liaison with the devolved administrations – Welsh Government, National Records of Scotland (NRS) and the Northern Ireland Statistics and Research Agency (NISRA). Information on our assumption-setting process can be found in National population projections, how the assumptions are set: 2018-based.

This report contains background information for the 2018-based national population projections. This includes:

• defining the base population
• the method of projection
• background on principal and variant projections
• summary of the long-term assumptions of future levels of fertility, mortality and migration
• the datasets available
• the changes to State Pension age

Definition

We use estimates of the usually resident population of the UK and its constituent countries in mid 2018 as our starting population. The usually resident population is defined by the standard United Nations definition for population estimates, and it includes people who reside in the area for a period of at least 12 months whatever their nationality. Members of HM Armed Forces in the UK are included, but members of HM Armed Forces and their families who are abroad are excluded. Members of foreign armed forces in the UK are included, with any accompanying dependants.

Base populations for individual countries

We base the projections for England and Wales on the mid-2018 population estimates published by the Office for National Statistics (ONS) on 26 June 2019. The projections for Scotland are based on the Mid-2018 population estimates Scotland published by National Records of Scotland (NRS) on 25 April 2019, and the projections for Northern Ireland are based on the 2018 Mid Year Population Estimates for Northern Ireland published by the Northern Ireland Statistics and Research Agency (NISRA) on 26 June 2019. Population estimates use the 2011 Census as the starting population and then update these annually to account for population change.

Estimates of the population aged 90 years and over

We prepare official mid-year population estimates by individual age to the age of 89 years, with an upper age band for all those aged 90 and over. We produce estimates of the population aged 90 to 104 years by single year of age and for the 105 years and over age group using the Kannisto–Thatcher survivor ratio method, controlling the results to agree with the official estimates of all those aged 90 years and over.

We produce projections for successive years running from one mid year to the next. For each age, we take the starting population and then account for net migration less the number of deaths to produce the number in the population, one year older, at the end of the year. We then add survivors of those born during the year. Age is defined as completed years at the last birthday.

We assume migration occurs evenly throughout the year. For computing purposes, this is equivalent to assuming that half the migrants in a given year at a given age migrate at the beginning of the year and half at the end of the year. The number of net migrants we add to obtain the population aged x plus 1 at the end of the projection year therefore consists of half of those migrating during the year at age x and half of those migrating during the year at age x plus 1.

We obtain the number of deaths in a year by adding half of the net inward migrants at each age to the number in the population at the beginning of the year and applying the mortality rate q_x+1/2, which is the probability of death between one mid-year period and the next. The mortality rates we use in the projections represent the probabilities of death between one mid-year period and the next, according to a person’s age at their last birthday at the beginning of the period. We also give the appropriate rate of infant mortality (that is, the probability of a new-born child not surviving until the following mid-year). This is about 85% of the full, first year of life infant mortality rate more generally used in official statistics.

We calculate the number of births in the year by multiplying the average number of women at each single year of age during the year (taken as the mean of the populations at that age at the beginning and end of the year) by the fertility rate applicable to them during that year. We assume the total number of births in a year is divided between the sexes in the ratio of 105 males to 100 females, in line with recent experience. We calculate the number of infants aged zero at the end of the year by taking the projected number of births and deducting the number of deaths, which is found by applying the infant mortality rate and adding half the number of net migrants aged zero at their last birthday.

We compute the principal projections for each of the constituent countries of the UK and add together the results to produce projections for England and Wales, Great Britain, and the UK.

We use the long-term assumptions of future fertility, mortality and net migration (that is, immigrants minus emigrants), summarised in Table 2, in the 2018-based principal projections. We agree the long-term assumptions in consultation with the Northern Ireland Statistics and Research Agency (NISRA), National Records of Scotland (NRS) and Welsh Government. Table 2 also compares assumptions for the 2018-based projections with the assumptions for the previous 2016-based projections.

For the UK, the assumed average completed family size is 1.78 children per woman by mid 2043, 0.06 lower than the 2016-based projections for the same year. It increases slightly to close to 1.79 later in the projection. More information can be found in National population projections, fertility assumptions: 2018-based.

Assumptions on improvements in principal mortality are broadly unchanged from the 2016-based projections. For the 2018-based projections, the assumption is that annual rates of mortality improvement would converge to 1.2% for ages 0 to 90 years by 2043 (the 25th year of the 2018-based projections) and remain constant thereafter. This is for both males and females, for all constituent countries of the UK. For ages above 90 years, annual improvement rates are set to decline from 1.2% to 0% between ages 91 and 110 years. For ages above 110 years, a 0% improvement rate is assumed.

Although we have not changed our assumptions about the long-term rate of improvement in life expectancy for most ages, actual life expectancy has increased less than projected since mid 2016. This means in each year of the 2018-based projections, the projected life expectancy is lower than in the 2016-based projections. More information can be found in National population projections, mortality assumptions: 2018-based.

The new long-term assumption for net international migration to the UK is +190,000 each year compared with +165,000 each year in the 2016-based projections. We calculate cross-border migration (moves between countries of the UK) by applying rates of movement between each pair of countries to the population by age and sex. The rates are derived as an average of the last five years’ estimates (2014 to 2018). 'More information can be found in National population projections, migration assumptions: 2018-based.

We have published projections to 100 years ahead. For each country and variant combination, we have made two summary tables and a zipped open data file (XML format) available to download.

The first summary table contains the total projected population for all years of the projection, the components of change and other summary statistics.

The second summary table contains the projected population in five-year age groups for all years of the projection.

The XML open data files contain:

• population by single year of age (0 to 104 years), age groups (105 to 109 years, 110 years and over) and sex
• fertility assumptions by single year of age of mother (15 to 46 years)
• mortality assumptions by single year of age (0 to 125 years) and sex
• cross-border rates for each country flow by single year of age (0 to 125 years) and sex
• births by age of mother (15 to 46 years)
• deaths by age (0 to 105 years and over) and sex
• in, out and net cross-border migration by age (0 to 105 years and over) and sex
• in, out and net international migration by age (0 to 105 years and over) and sex
• in, out and net total migration by single year of age (0 to 105 years and over) and sex

Pensionable ages for men and women

Since 2010, the State Pension age has been increasing. By 2020, it will change from 65 years for men and 60 years for women, to 66 years for both sexes. State Pension age will then increase to 67 years for both men and women between 2026 and 2028. Under the current law, State Pension age is due to increase to 68 years between 2044 and 2046.

The proportions used to calculate the population of working age and pensionable age, along with a worked example of how these proportions are applied, are available in the Calculating State Pension age: Pensions Act 2014 dataset.

Full details about the current and planned changes to the State Pension age under the Pensions Acts of 1995, 2007, 2011 and 2014 can be found in the following.

Changes to State Pension age

The following tables show how the legislated increases in State Pension age will be phased in. The published national projections output tables include the projected number and percentage of those of working age and pensionable age based on the phasing detailed in this section.

Sources: Pensions Act 1995, Chapter 26, Part II, Section 126 and Schedule 4; Pensions Act 2007, Chapter 22, Part I, Section 13 and Schedule 3; Pensions Act 2011, Part 1, Section 1 and Schedule 1; Pensions Act 2014, Part 3, Section 26 and Section 27.