There are 43,560 square feet to an acre. Since an acre is a measure of an area, not length, it is defined in square feet. An acre can be any shape, square, rectangle, triangle, or even a circle as long as the total land area is 43,560 square feet.
Other measures equivalent to an acre are:
1 acre = 43,560 square feet
1 acre = 4046.86 square meters
1 acre = 4840 square yards
1 acre = 0.404686 hectares
1 acre = 1/640th of a square mile
The definition of an acre by modern surveyors is 1/640th of one square mile, or 640 acres per square mile, or “section”. Since there are 640 acres in one square mile, to calculate how many acres are in a large area of land just multiply the square miles by 640.
What is a commercial acre?
A real estate term, the commercial acre was invented by realtors in the United States for use in large cities, and is a legal unit in some US states. It is the portion of property zoned for commercial use after allowances for roads, sidewalks, setbacks, open spaces and other unsuitable areas. The commercial acre is a rounded figure calculated at 82.6 percent of an international (residential) acre.
1 commercial acre = 36,000 square feet
1 commercial acre = 4000 square yards
1 commercial acre = 3342.8 square meters
The history of an acre
The word “acre” is derived from Old English æcre originally meaning “open field”. The acre was originally originally an English unit of measurement defined as the rough amount of land that was tillable by a yoke of oxen in one day (the yoke is the wooden beam used between a pair of oxen or other animal), which explain thee definition of the area of a rectangle as “one chain and one furlong” sides of length.
The most standard shape for an acre is 660 feet by 66 feet (43,560 square feet), or one ‘furlong’ by one ‘chain’.
The old English measurement terms were:
Rod: A farming instrument used to spur the oxen pulling the plough, a rod was approximately about 5.5 yards in length.
Chain: The chain is a unit of length equal to 66 feet, or 22 yards (4 rods), and is subdivided into 100 (app. 7.92 in.) chain links. The chain was the standard English surveyor’s measure (the chain and link became standard English surveyors’ units of length and crossed to the colonies). 1 Chain = 100 links or 66 feet.
Furlong: There are 10 chains in a furlong, 220 yards or 660 feet. 660 feet is the length of a furrow, which was the expected length a ploughing team could work during the day without needing a break.
Acre: The amount of land tillable by one man behind one ox in one day.
For more information on the history of the acre please visit WikiPedia