Table of Contents
The data sources, definitions, and methods used to count and track migration flows and the migrant population.
Net migration
The difference between the number of people arriving in the UK to live (immigration) and the number leaving (emigration), over a given period. It is the headline figure in UK migration debate. Net migration was estimated at 204,000 in the year ending June 2025 — sharply lower than the record 944,000 in early 2023, driven by falls in non-EU work and study migration.
Immigration
The arrival of people in the UK to live, typically for at least 12 months. In the year ending June 2025, total long-term immigration was estimated at 898,000. Immigration is distinct from short-term arrivals such as visitors. It is measured by the ONS using a combination of border data, visa records, and administrative sources.
Emigration
The departure of people from the UK to live elsewhere, typically for at least 12 months. In the year ending June 2025, emigration was estimated at 693,000. It includes both British nationals and foreign nationals. The majority of those emigrating had originally arrived on study-related visas.
Long-Term International Migration (LTIM)
The ONS statistical series estimating the flow of long-term migrants to and from the UK. It uses the UN definition of a long-term migrant — someone who changes their country of usual residence for 12 months or more. The methodology has been substantially revised since 2021 to make greater use of administrative data, and estimates are currently classified as official statistics in development.
Migration stocks
A measure of the total number of migrants living in a country at a given point in time, as opposed to flows (arrivals and departures). In the UK, stocks are typically measured by country of birth or nationality. As of mid-2023, approximately 11.4 million people living in England and Wales were born abroad, representing around 19% of the population.
Migration flows
The number of people moving to or from a country over a given period. Flows are distinct from stocks. The UK’s net migration figure is derived from flows. Measuring flows accurately is challenging because it requires tracking both arrivals and departures, and intentions do not always match behaviour.
International Passenger Survey (IPS)
A long-running ONS survey of passengers entering and leaving the UK at ports and airports. It was the primary source of UK net migration data before the COVID-19 pandemic. The IPS recorded intentions, not outcomes, and missed significant migration flows — particularly EU nationals at regional airports. It was scaled back and is now used mainly for British national estimates and reasons for migration.
Registration and Population Interaction Database (RAPID)
A Department for Work and Pensions database combining tax and benefit records for anyone holding a National Insurance number. The ONS now uses RAPID as a core data source for estimating EU and British national migration. It measures “activity” in the UK system but cannot capture people — such as children and non-working family members — who do not interact with tax or benefit records.
Migration Watch System (MWS)
A linked Home Office database combining visa, border entry and exit, and immigration status records to build travel histories for non-EEA nationals. Used by the ONS to estimate long-term immigration and emigration of non-EU migrants. It enables the tracking of individuals’ entries and exits alongside their visa and status information.
Admin-based migration estimates (ABMEs)
The ONS’s emerging methodology for producing migration statistics using administrative data — such as tax records, benefit data, and border crossings — rather than survey data. ABMEs are currently classified as official statistics in development, meaning they are published but subject to ongoing revision as methods are refined.
National Insurance number (NINo) allocations
The number of National Insurance numbers issued to non-UK nationals registering for work in the UK. NINo data was historically used as a proxy for labour migration flows. It is now less prominent as a standalone measure, having been largely superseded by visa data and administrative estimates in the official migration statistics framework.
Country of birth
One of the two main ways of defining the migrant population in official statistics — counting people whose place of birth differs from their country of residence. Unlike nationality, it is a fixed characteristic. The Census records country of birth, and it underpins estimates of the foreign-born population, which stood at approximately 11.4 million in England and Wales as of mid-2023.
Nationality
The second main way of defining the migrant population in statistics — counting people who hold the citizenship of a different country from the one in which they live. Unlike country of birth, nationality can change through naturalisation. The ONS uses both country of birth and nationality to describe the non-UK population in different contexts.
Non-UK born population
People living in the UK who were born in another country, regardless of their current nationality. This is the standard statistical measure of the resident migrant population. It includes people who have since become British citizens. As of mid-2023, the non-UK born population of England and Wales was approximately 11.4 million.