How city area is measured in the United Kingdom

What "city" means in the United Kingdom

In the UK, the word "city" does not name a level in the official statistical geography, so it tells you little about where a boundary runs. Official statistics are organised by local government units instead. The ONS publishes Standard Area Measurements for each local authority district, county or unitary authority, region and country, and those are the figures to use when you need an official area.

London shows why the word alone is not enough. The City of London is a single local authority district with its own corporation, the City of London Corporation, which gives its resident population as 8,600. Greater London is a much larger unit: the London Government Act 1963 defines it as the areas of the London boroughs, the City and the Temples combined, and the Greater London Authority governs it today. The ONS publishes Greater London as the region "London", and the GLA projects its 2025 population at 9,650,677. In ONS regional statistics, "London" therefore means Greater London, and on 17 October 2026 a Nominatim search for London returned Greater London too (see below); a statistic labelled "City of London" covers that one district only.

The ONS also publishes Built-up Areas, which trace the urban footprint from land use and are drawn independently of council boundaries. We did not find an official published area for London's Built-up Area, so this page uses only the local-government units.

Why our map may show a different number

When we have no stored boundary for London, the app sends OpenStreetMap's Nominatim service the search "London, United Kingdom" restricted to cities. On 17 October 2026 that search returned the Greater London relation (OpenStreetMap relation 175342, administrative level 5, tagged as the jurisdiction of the Greater London Authority). When no boundary is stored and the result is a relation with its own polygon, the app draws that polygon directly and does not mark it approximate. A search for "City of London, United Kingdom" returned the City of London relation (51800), also a full polygon.

When we have no stored area for a boundary, the map calculates the area from the polygon, not from ONS. For both London units, the OpenStreetMap polygon follows the outer boundary into the Thames, so its area is close to the ONS total extent (1,595 km²^3,4 for Greater London) and larger than the ONS land-only figure (1,572 km²^5,6). The polygon areas and the ONS figures still differ slightly.

Problems getting boundaries in the United Kingdom

The first problem is water. Every ONS Standard Area Measurement record carries several measures for the same unit: the total extent (the "extent of the realm", which for London includes the tidal Thames), the area to mean high water, the inland water, and the land area. For a unit that touches the tidal river these differ, so two correct-looking numbers can describe the same council. In the City of London, the gap between the total extent and the area to mean high water is the river itself. For Greater London, the total extent is 1,595 km²^3,4 and the land area is 1,572 km²^5,6. Always check which measure a published figure uses before comparing it with another.

The second problem is publishers. The Greater London Authority publishes its own borough areas, taken from Ordnance Survey boundaries, and they differ slightly from the ONS figures, by well under one percent. Both are official; they are published separately by different publishers. This page cites ONS as the primary figure and shows the GLA number alongside it.

The third problem is OpenStreetMap itself. It holds two "Greater London" boundaries: the Greater London Authority area, which includes the City, and the ceremonial county, which by its own description excludes the City of London. Wikidata links Greater London to the ceremonial county, so a tool that follows Wikidata can draw a Greater London with a hole where the City should be. We draw the Greater London Authority area, which matches the ONS region.