Understanding our future population: Why projections are not predictions

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There are many reasons for wanting to know the size of the future population of the UK. How many school places will we need? How many hospitals? How many people will claim a State Pension? But looking into the future is challenging. As James Robards explains, our population projections take into account current and past trends, and as those patterns change, we adjust our projections accordingly.

Population projections tell us about the possible future population size. These projections provide an age and sex break down for the UK. Providing information about what might happen in the future assists policy makers, including HM Treasury, in their decision-making.

Our assumptions are based on current and past demographic behaviours (births, deaths and migration) and trends. They are not forecasts and do not attempt to predict the possible effects of any future political or economic developments.

Using the latest information available to us, today’s projections suggest that the UK’s population is likely to grow faster than we previously published, reflecting our most recent estimates of international migration and our new assumptions about future international migration.

Unprecedented international migration

We are living in an ever-changing world. In setting our latest assumptions we have sought expert advice, which along with the latest data, point towards the likelihood of higher levels of international migration over the long-term than in our previous releases.

When we published our last projections in January 2023 there was one provisional ‘high’ net migration data point of 504,000 to the year mid-2022. This was taken in the midst of many unique world events, including the war in Ukraine and the impact of the pandemic on the movement of people. We therefore increased our principal long-term assumption for net international migration to the UK to 245,000 (by 20% compared with our earlier release) from mid-2027.

Since then, we have seen a longer period of unprecedented levels of international migration.

Expert views and the latest data covering the last ten years has led us to develop a long-term net migration assumption of 315,000 each year from year ending mid-2028 onwards. It is important to recognise that there is uncertainty in the provisional international migration estimates. Future migration will be affected by policy changes as well as the impact of as yet unknown migrant behavioural patterns in the future. Put simply, if migration comes down so will future projections.

If net international migration were to be, say, 20% higher than our long-term assumption, then it would be 379,000 per year. If it were to be 20% lower than our assumption, then it would be 253,000 per year. That is why we call these projections and not forecasts. There is uncertainty and these differences would affect the total size of the population accordingly.

The impact of other data sources

Our last projections release in January 2023 used the latest data at that point. Throughout the last 12 months a wide range of new data have become available with the potential for use in making population projections. These include information from the Census 2021 reconciliation and rebasing processes (England, Wales and Northern Ireland), estimates of international migration 2012-2021 and the latest provisional estimates of international migration for mid-2022 and mid-2023.

Our latest release today of 2021-based interim national population projections incorporates as much of this as possible so our statistics on what the future might look like reflect the latest data.

Furthermore, we have incorporated the births and deaths data for mid-2022 and mid-2023 so we reflect the latest trends for these years even though we are reusing the same assumptions as we developed for the 2020-based projections.

Looking to the future

Throughout 2024 we will, as usual, be releasing new data on a range of areas which will in some form contribute towards the development of assumptions for population projections. We will use these statistics, along with expert advice, to develop new assumptions for all components of population change (fertility, migration and mortality) with a mid-2022 base. That projections release is planned for October or November 2024. It will also use census data for Scotland, as well as the latest data on international migration.

To help us in this work and on our work on subnational population projections and household projections we are also starting a user engagement exercise at consultations.ons.gov.uk. We are asking for feedback on plans for the upcoming releases at the end of the year and early part of 2025. We provide numerous tables and articles as part of our releases and want to ensure we understand the needs of all our users. For these releases we will also plan to show the uncertainty in future demographic trends through the release of a range of variant projections and welcome feedback on your need for these.

James Robards

James Robards is Head of Population and Household Projections