Square Mile (mi²)
The square mile, or mi², is the area of a square one mile on each side, or 640 acres. Its history dates back to the mile, which originated in Roman times. The square mile became a set measurement in England. It is still widely used in the US and the UK for large areas, such as cities, counties, and parks. Most countries use square kilometers, but in English-speaking regions, the square mile remains common, especially in historical and legal contexts.
Acre (ac)
The acre, or ac, is a unit of area used in the imperial and US systems. It originated in medieval England and referred to the land a team of oxen could plow in a day. Today, one acre is 4,840 square yards or 4,046.856 square meters. Acres are still used in the US, Canada, and the UK for real estate, land sales, and farms. While the metric system is more common worldwide, the acre is still used because it is part of tradition and is easier to use in daily life.