Square Mile (mi²)
Modern equivalent
640 acres = 2.59 square kilometres
The largest customary imperial area unit, containing exactly 640 acres.
Because a square mile divides cleanly into 640 acres, it became the natural unit for describing counties, water catchments, moorland and national park boundaries. British administrative and Ordnance Survey figures were quoted in square miles well into the twentieth century, and the unit persists in public discussion of land area even where official statistics have moved to hectares.