An acre is among the oldest units applied across the world to measure vast tracts of land, and has undergone a lot of changes along the way. Most traditionally used throughout the British Empire, the Anglo-Saxon acre was defined as a strip of land 1 × 1/10 furlong, or 40 × 4 rods (660 × 66 feet).
Used in the imperial system of units and the US system, the modern acre is equal to 4,840 square yards, 43,560 square feet, 4,047 square metres and 0.4047 hectares. An acre could also be defined as an area one furlong long by 4 rods wide.
Note here that imperial units are systems of weights and measures used officially in the Great Britain from 1824 until the adoption of the metric system beginning in 1965. Now defined in metric terms, these units also are part of the US Customary System, which derived from the British Imperial System.
4,840 square yards
43,560 square feet
4,047 square metres
Derived from the ancient English word, aker which denotes 'open field', acre as a unit of measure has no prescribed shape. While the acre usually goes by the full name, the abbreviation “ac” is also for sometimes as the symbol of this unit.
Acre is widely used all across the world as a land measurement unit, including Canada, Jamaica, the US, Spain, France, Germany and most of South Asia, including India, Pakistan, Sri Lanka and Bangladesh. However, it is the UK where the unit is more commonly used. In the middle age in England, an acre of land was defined as an area that could be ploughed in one day by a yoke of oxen. Such has been the popularity of this measurement unit across the British Empire that the size of farms usually expressed in acres even if expressing them in square miles made better sense because of the large numbers involved.
Note here that there is no standard measurement of an acre, and it does not denote the same area in countries where it is used. While an acre of land in Scotland is equivalent to 1.27 standard acres, the Irish acre is equal to 1.6 British or US acres.
In India, residential plots are measured in square feet while agricultural land is measured in acres or hectare. However, bigha is a unit more locally used in the northern parts of the country. Since bigha denotes different measurements in different states, the bigha-acre conversion would show varying values, depending on the state where the land is situated.