---
title: "UK mid-year population estimates"
method: GET
path: "/api/v1/ons/population"
tags: ["Statistical Agencies"]
---

# UK mid-year population estimates

`GET /api/v1/ons/population`

UK mid-year population estimates from Office for National Statistics. Totals for the United Kingdom (UKPOP), England (ENPOP), Wales (WAPOP), Scotland (SCPOP), and Northern Ireland (NIPOP). Annual, released in summer.

## Query parameters

- `limit` integer — Number of most-recent annual observations per country.

## Response `200`

Annual mid-year population estimates for UK countries.

- EnvelopeOnsPopulationPayload
  - `data` OnsPopulationPayload, required
    - `uk` OnsSeries, required — One curated ONS CDID series, unified shape regardless of parent dataset.
      - `cdid` string, required — ONS time series identifier (stable 4-char code, e.g. 'L55O', 'MGSX').
      - `title` string, nullable — Series title as published by ONS.
      - `unit` string, nullable — Unit of measure (e.g. '%', 'index 2015=100', 'GBP m', 'persons').
      - `frequency` string, required — Canonical frequency returned in `observations` ('monthly', 'quarterly', 'annual').
      - `observations` OnsObservation[], required — Observations at the canonical frequency, ordered oldest to newest.
        - `period` string, required — ISO 8601 start-of-period (YYYY-MM-DD). Months = first day of month, quarters = first day of quarter, years = January 1.
        - `period_label` string, required — Original ONS label, e.g. '2026 FEB', '2025 Q4', '2024', or rolling LFS label '2025 NOV-JAN'.
        - `value` number, nullable — Numeric observation value. Null if upstream reported an empty cell.
        - `frequency` string, required — Frequency of this row: 'monthly', 'quarterly', or 'annual'.
        - `update_date` string, nullable — ONS updateDate for the observation (ISO 8601 UTC).
      - `rollups` object — Optional higher/lower frequency rollups keyed by 'annual', 'quarterly', or 'monthly'.
    - `england` OnsSeries, required — One curated ONS CDID series, unified shape regardless of parent dataset.
      - `cdid` string, required — ONS time series identifier (stable 4-char code, e.g. 'L55O', 'MGSX').
      - `title` string, nullable — Series title as published by ONS.
      - `unit` string, nullable — Unit of measure (e.g. '%', 'index 2015=100', 'GBP m', 'persons').
      - `frequency` string, required — Canonical frequency returned in `observations` ('monthly', 'quarterly', 'annual').
      - `observations` OnsObservation[], required — Observations at the canonical frequency, ordered oldest to newest.
        - `period` string, required — ISO 8601 start-of-period (YYYY-MM-DD). Months = first day of month, quarters = first day of quarter, years = January 1.
        - `period_label` string, required — Original ONS label, e.g. '2026 FEB', '2025 Q4', '2024', or rolling LFS label '2025 NOV-JAN'.
        - `value` number, nullable — Numeric observation value. Null if upstream reported an empty cell.
        - `frequency` string, required — Frequency of this row: 'monthly', 'quarterly', or 'annual'.
        - `update_date` string, nullable — ONS updateDate for the observation (ISO 8601 UTC).
      - `rollups` object — Optional higher/lower frequency rollups keyed by 'annual', 'quarterly', or 'monthly'.
    - `wales` OnsSeries, required — One curated ONS CDID series, unified shape regardless of parent dataset.
      - `cdid` string, required — ONS time series identifier (stable 4-char code, e.g. 'L55O', 'MGSX').
      - `title` string, nullable — Series title as published by ONS.
      - `unit` string, nullable — Unit of measure (e.g. '%', 'index 2015=100', 'GBP m', 'persons').
      - `frequency` string, required — Canonical frequency returned in `observations` ('monthly', 'quarterly', 'annual').
      - `observations` OnsObservation[], required — Observations at the canonical frequency, ordered oldest to newest.
        - `period` string, required — ISO 8601 start-of-period (YYYY-MM-DD). Months = first day of month, quarters = first day of quarter, years = January 1.
        - `period_label` string, required — Original ONS label, e.g. '2026 FEB', '2025 Q4', '2024', or rolling LFS label '2025 NOV-JAN'.
        - `value` number, nullable — Numeric observation value. Null if upstream reported an empty cell.
        - `frequency` string, required — Frequency of this row: 'monthly', 'quarterly', or 'annual'.
        - `update_date` string, nullable — ONS updateDate for the observation (ISO 8601 UTC).
      - `rollups` object — Optional higher/lower frequency rollups keyed by 'annual', 'quarterly', or 'monthly'.
    - `scotland` OnsSeries, required — One curated ONS CDID series, unified shape regardless of parent dataset.
      - `cdid` string, required — ONS time series identifier (stable 4-char code, e.g. 'L55O', 'MGSX').
      - `title` string, nullable — Series title as published by ONS.
      - `unit` string, nullable — Unit of measure (e.g. '%', 'index 2015=100', 'GBP m', 'persons').
      - `frequency` string, required — Canonical frequency returned in `observations` ('monthly', 'quarterly', 'annual').
      - `observations` OnsObservation[], required — Observations at the canonical frequency, ordered oldest to newest.
        - `period` string, required — ISO 8601 start-of-period (YYYY-MM-DD). Months = first day of month, quarters = first day of quarter, years = January 1.
        - `period_label` string, required — Original ONS label, e.g. '2026 FEB', '2025 Q4', '2024', or rolling LFS label '2025 NOV-JAN'.
        - `value` number, nullable — Numeric observation value. Null if upstream reported an empty cell.
        - `frequency` string, required — Frequency of this row: 'monthly', 'quarterly', or 'annual'.
        - `update_date` string, nullable — ONS updateDate for the observation (ISO 8601 UTC).
      - `rollups` object — Optional higher/lower frequency rollups keyed by 'annual', 'quarterly', or 'monthly'.
    - `northern_ireland` OnsSeries, required — One curated ONS CDID series, unified shape regardless of parent dataset.
      - `cdid` string, required — ONS time series identifier (stable 4-char code, e.g. 'L55O', 'MGSX').
      - `title` string, nullable — Series title as published by ONS.
      - `unit` string, nullable — Unit of measure (e.g. '%', 'index 2015=100', 'GBP m', 'persons').
      - `frequency` string, required — Canonical frequency returned in `observations` ('monthly', 'quarterly', 'annual').
      - `observations` OnsObservation[], required — Observations at the canonical frequency, ordered oldest to newest.
        - `period` string, required — ISO 8601 start-of-period (YYYY-MM-DD). Months = first day of month, quarters = first day of quarter, years = January 1.
        - `period_label` string, required — Original ONS label, e.g. '2026 FEB', '2025 Q4', '2024', or rolling LFS label '2025 NOV-JAN'.
        - `value` number, nullable — Numeric observation value. Null if upstream reported an empty cell.
        - `frequency` string, required — Frequency of this row: 'monthly', 'quarterly', or 'annual'.
        - `update_date` string, nullable — ONS updateDate for the observation (ISO 8601 UTC).
      - `rollups` object — Optional higher/lower frequency rollups keyed by 'annual', 'quarterly', or 'monthly'.
  - `meta` SugraMeta, required — Metadata attached to every /api/v1/* response envelope.
    - `endpoint` string, required — Requested endpoint path.
    - `data_time` string, required — ISO 8601 UTC timestamp of the source data, not of the request.
    - `response_time` string, required — ISO 8601 UTC timestamp when this response was produced.
    - `provider` string, required — API name and version.
    - `source` string, nullable — Identifier of the primary upstream source used for this response.
    - `attribution` string, nullable — Human-readable attribution mandated by an upstream source (e.g. a securities regulator or self-regulatory organization). Present only on responses whose source requires the owner and source to be clearly identified. Do not remove or alter it when using the response.
    - `fallback_used` boolean, nullable — True when the primary source failed and a fallback produced the data.
    - `fallback_chain` string[], nullable — Ordered list of sources attempted, in the order they were tried.
    - `cached` boolean, nullable — True when this response was served from the internal cache.
    - `stale` boolean, nullable — True when the cached response was returned after the upstream rate-limited or errored. Clients can use this to detect degraded data.

## Other responses

- `401` — Missing or invalid `x-api-key` header. JSON body with a stable `code` distinguishing `missing_api_key` (no header sent) from `invalid_api_key` (header sent, key not accepted); any other 401 source carries the generic `unauthorized` with its detail as `reason`. Plus `hint`. `plan` is always null on 401 - an unauthenticated request has no plan; quota exhaustion is 429, not 401.
- `422` — Validation Error
- `429` — Daily rate limit exceeded. Check `X-RateLimit-Reset` for the next window.
- `503` — Upstream source is temporarily unavailable. Retry after a short delay.

---

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