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The Square Mile (symbol: mi² or sq mi) is a unit of area within the British Imperial and United States customary systems. It is defined as the area of a square whose sides each measure exactly one statute mile (mi) in length.

Key relationships within the imperial/US customary system:

• 1 mi² = 1 mile × 1 mile
• 1 mile = 1,760 yards (yd) = 5,280 feet (ft)
• Therefore, 1 mi² = (5,280 ft)² = 27,878,400 square feet (ft²)
• 1 mi² = (1,760 yd)² = 3,097,600 square yards (yd²)
• 1 mi² = 640 acres (ac) (This is a fundamental definition within the system)

In terms of metric units:

• 1 mile = 1.609344 kilometers (km) exactly (based on the international yard)
• Therefore, 1 mi² = (1.609344 km)² ≈ 2.589988 square kilometers (km²)
• 1 mi² ≈ 259 hectares (ha)

• Origin (The Mile): The square mile is derived from the mile, a unit of length whose name comes from the Roman mille passus ("thousand paces"). The length of the mile varied considerably until the statute mile was defined by an English Act of Parliament in 1593 as 5,280 feet.
• Imperial System: The square mile became the standard unit for measuring large land areas within the British Empire. Its definition based on the acre (640 acres per square mile) was particularly important for land surveying and administration, especially in the colonization of North America.
• US Public Land Survey System (PLSS): The square mile, referred to as a section, is a fundamental unit in the PLSS, which was established by the Land Ordinance of 1785. This system was used to survey and divide vast amounts of land in the United States. Townships are typically defined as 36 square miles (a 6x6 grid of sections).
• Standardization (International Mile): While the statute mile was legally defined, its exact relationship to metric units was solidified with the international agreement of 1959, which defined the yard (and thus the foot and mile) in terms of the meter. This fixed the conversion to square kilometers.
• Continued Use: Despite global metrication, the square mile remains the primary unit for large land areas and geographical measurements in the United States and is still used in some contexts in the UK, Canada, and other countries with historical ties to the imperial system.

The square mile is used for measuring large areas, particularly land:

• Geography and Cartography (US, UK, etc.): Measuring the area of cities, counties, states (in the US), national parks, large lakes, islands, and administrative regions in countries using imperial/US customary units.
• Demography (US, UK, etc.): Calculating population density, typically expressed as inhabitants per square mile (people/mi²).
• Land Surveying (US): A fundamental unit (section) in the Public Land Survey System used for defining property boundaries over large parts of the United States.
• Real Estate (Large Tracts): Sometimes used for very large rural properties or ranches, although acres are often preferred.
• Historical Documents: Common in older maps, land deeds, and geographical descriptions in relevant countries.
• Environmental Science: Measuring the extent of large reserves, habitats, or areas affected by large-scale phenomena (e.g., wildfire extent) in relevant regions.

It is generally too large for measuring individual residential properties (acres or square feet are used) or room sizes.