About the Census data
How the constituency-level census layers on this site are sourced, aggregated, and interpreted.
This map shows demographic data from the most recent UK Censuses, aggregated to the 2024 Westminster parliamentary constituency boundaries introduced for the July 2024 general election.
What censuses are used
Because the UK censuses are run by different statistical agencies on slightly different timetables, the data on this map combines three sources:
- England and Wales: Census 2021 — conducted on 21 March 2021 by the Office for National Statistics (ONS).
- Northern Ireland: Census 2021 — conducted on 21 March 2021 by the Northern Ireland Statistics and Research Agency (NISRA).
- Scotland: Census 2022 — conducted on 20 March 2022 by the National Records of Scotland (NRS), postponed by a year because of the Covid-19 pandemic.
Where the map shows a national comparator, it uses the UK-wide figure where available, or the England & Wales figure for topics where UK aggregation isn't published.
How the data reaches 2024 boundaries
The 2024 constituency boundaries were introduced by the Boundary Commissions in their 2023 review. Because they differ from the boundaries used in earlier censuses, the House of Commons Library reaggregates the underlying small-area census data (Output Areas and their equivalents in each nation) onto the new constituencies and publishes constituency-level tables.
The map and seat pages on this site pull from those Commons Library publications.
Topics currently available
- Ethnicity — broad ethnic groups (White, Asian, Black African or Caribbean, Mixed or Multiple ethnic groups, Other ethnic group).
- Country of birth — grouped by UK, European Union, and Rest of world.
- Religion — broad religion groups (Christian, Muslim, Hindu, Sikh, Jewish, Buddhist, No religion, Other, Not stated).
Further topics — age, qualifications, tenure, economic activity, and household composition — will be added as those Commons Library datasets are incorporated.
Historical comparison
The 2024 Westminster boundaries are new, so older censuses are not natively published on them. To make historical comparison possible, small-area data from earlier censuses is reaggregated onto the 2024 boundaries. 1991, 2001, 2011, and 2021 are all available via the year selector on the map, with side-by-side change tables on each seat's panel.
2011 Census data methodology
2011 ethnicity data on this map is reaggregated from 2011 Output Areas onto 2024 Westminster constituency boundaries. Output Areas are small ONS statistical units of roughly 125 households each. Because each 2024 constituency is built from hundreds of OAs, the reaggregation produces values very close to what a 2011 census run on 2024 boundaries would have produced.
The source data is resident counts (not percentages), so aggregation is a straightforward weighted sum. The only source of error is ONS's statistical disclosure control, which introduces rounding of less than 1 percentage point.
2011 data coverage on this site is currently England and Wales (around 573 constituencies). Scotland and Northern Ireland are not yet included - adding them requires equivalent OA-level data from the National Records of Scotland and NISRA, which will be added in a later release.
Year-over-year comparisons (2011 vs 2021) on a single seat's page use broad ethnic groups only - the sub-categories are not directly comparable between censuses because the 2011 and 2021 questionnaires used different sub-category structures. For example, "White: Gypsy or Irish Traveller" was introduced as a separate category in 2011, and "Roma" was added in 2021.
Earlier censuses
2001 and 1991 are shipped, reaggregated from their respective small-area geographies onto 2024 constituencies. 1981 remains on the roadmap and requires a further OA-to-PCON-2024 lookup at the quality and coverage levels ONS publishes.
Sources and further reading
- House of Commons Library — Constituency data: Ethnic groups
- House of Commons Library — 2021 census results
- Office for National Statistics — Census 2021
- National Records of Scotland — Scotland's Census 2022
- Northern Ireland Statistics and Research Agency — Census 2021
Caveats to keep in mind
- The Scottish data is from 2022 while England, Wales, and NI are from 2021. One year of drift across borders. For most metrics this is negligible.
- Constituency-level aggregation hides sub-constituency variation. A seat that appears ~50% White overall may contain neighbourhoods ranging from 15% to 85%. For sub-constituency detail, the ONS's LSOA and MSOA maps remain the better tool.
- Census data is self-reported. Broad ethnic group categories are the ones used by the ONS and are not directly comparable to some other international classifications.