Modelling assumptions and datasets used in the ONS's long-term population projections

Dyddiad y datganiad:
6 February 2026

FOI reference: FOI-2026-3364

You asked

Under the Freedom of Information Act 2000, I am requesting information regarding the modelling assumptions and datasets used in the ONS's long-term population projections (specifically those looking towards 2050 and 2100).

Please provide the following information:

1. Modelling Assumptions:

a. The specific "Net Migration" figures (annual average) assumed in the "High Migration" and "Principal" variants of the most recent 50-year population projections.

b. Any internal "Sensitivity Analysis" reports that model the impact on public service capacity (NHS, Housing, Education) if net migration continues at the 2024-2025 levels for more than 10 years.

2. Demographic Composition:

a. Does the ONS hold projections for the "Ethnic Composition" or "Country of Birth" breakdown of the UK population for the years 2050, 2063, and 2079? If so, please provide the data tables for these projections.

b. Any reports assessing the "Fiscal Impact" or "Incentive Effects" on tax revenue resulting from the projected change in the age-dependency ratio, specifically factoring in high-volume migration.

3. Governance and Methodology:

a. Have there been any internal reviews or "Quality Assurance" reports in the last 24 months regarding the accuracy of migration-related data sources (for example, International Passenger Survey vs. Administrative data)?

b. Any communications with the Home Office regarding the alignment of ONS population projections with actual Border Security Act outcomes.

We said

Thank you for your request for information regarding modelling assumptions and datasets used in the Office for National Statistics (ONS) long‑term population projections (towards 2050 and 2100), as well as migration‑related quality assurance and related analyses.

1) Modelling assumptions

1a) Net migration figures assumed in "High migration" and "Principal" variants (most recent 50‑year projections)

Our most recent national population projections are the 2022‑based national population projections (NPPs). The long‑term international migration assumptions for the UK used in the variants are published on the ONS website:

Key figures (UK long‑term assumptions used in the 2022‑based NPPs):

  • Principal long‑term net international migration: +340,000 per year (reached from year ending mid‑2028 onwards).
  • High migration variant long‑term net international migration: +525,000 per year.

These figures are as published in the previously mentioned migration‑assumptions article and underlying datasets.

We interpret your reference to the "most recent 50‑year population projections" as the 2022‑based NPPs (which project 50 years from the mid‑2022 base year). If you intended a different projection series, please let us know.

1b) Internal sensitivity analysis on public service capacity (NHS, housing, education) if 2024--2025 net migration levels persist >10 years

We do not hold this information. We have not produced internal reports or analyses modelling public service capacity impacts (for example, NHS, housing, education) based on migration levels persisting at 2024--2025 rates for ten years or more.

For further information, you may wish to consider contacting:

  • Department of Health and Social Care (DHSC) and NHS England for NHS capacity modelling.
  • Department for Levelling Up, Housing and Communities (DLUHC) for housing modelling/capacity.
  • Department for Education (DfE) for school places and education capacity analyses.
  • Office for Budget Responsibility (OBR) for macro‑fiscal scenarios and long‑term fiscal sustainability analyses that may indirectly consider demographics.

2) Demographic composition

2a) Projections by ethnic composition or country of birth for 2050, 2063, 2079

We do not hold projections of the UK population by ethnic group or by country of birth for these years within the national population projections. The 2022‑based NPPs provide totals by age and sex (with variant assumptions for fertility, mortality and migration) but do not include breakdowns by ethnicity or country of birth.

You may find the following the following relevant (not held within the NPPs):

  • Census 2021 (for observed, not projected, ethnic group and country of birth) and related analyses on the ONS website and Nomis.
  • Academic or departmental research may produce experimental projections by ethnicity or country of birth; these are outside ONS's NPP suite.

2b) Reports on fiscal impact or incentive effects on tax revenue from changes in the age‑dependency ratio, factoring high migration

We do not hold such reports within the NPP work. We do not produce fiscal impact or incentive‑effect assessments as part of the national population projections. As signposting:

  • The Office for Budget Responsibility (OBR) publishes Fiscal risks and sustainability reports and Economic and fiscal outlooks that assess long‑term fiscal pressures and demographics (not ONS products).
  • HM Treasury and relevant departments may hold fiscal analyses relating to demography and migration.

3) Governance and methodology

3a) Internal reviews or quality assurance on migration‑related data sources in the last 24 months

We have undertaken substantial quality assurance and method development relating to the transition from the International Passenger Survey (IPS) to administrative data sources for long‑term international migration (LTIM) estimates. These have been published on our website:

These articles describe our move to Home Office (HO) and Department for Work and Pensions (DWP) administrative data for different nationality groups, the associated quality assurance, and how these changes feed into improved migration estimates. They are the most relevant and up‑to‑date QA materials for migration sources over the last two years. Where applicable, additional Quality Assurance of Administrative Data (QAAD) materials are linked from these articles or topic pages.

3b) Communications with the Home Office about aligning ONS population projections with Border Security Act outcomes

We do not hold records of communications with the Home Office on aligning ONS population projections with outcomes of the Border Security Act. The national population projections use published methodological assumptions for fertility, mortality and migration and are not aligned to legislative "outcome targets".

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