In the English-speaking world, before 8th – 21st century, a unit of land area, now
A square plot of ground, 208.7 feet on a side, will cover an acre. An American football field, 360 feet by 160 feet, is about 1.3 acres; 12 high school basketball courts are a little more than 1 acre.
The acre was also used as a unit of tax assessment rather than of land area as such. See definition 5 below.
In 1979, Council Directive 80/181/EEC of the European Community1, governing standardization on metric units in the European Union, included an exception that permitted Ireland and the United Kingdom to continue using the acre for a limited time. The Council was supposed to set an end date by 31 December 1989. In 1989, the directive was amended to leave the setting of the date to the U.K. and Ireland. Finally in 2007 the exception was allowed to expire2, since Ireland had finished converting its land registration system to meters by the end of 1998, and the U.K. sometime afterwards. The acre is no longer legal in either country.
In the United States, since the acre is a land measure it is currently based on the U.S. survey foot and not on the international foot. One acre is about 4,046.873 square meters.
The acre was originally the amount of land that could
be plowed in a single day with oxen 
, or actually, what could be done by midday,
since refueling took all afternoon (the oxen had to be put out to pasture).
Similar units of land area are found wherever animals are used for plowing; the
German Morgen and Roman jugerum had much the same meaning.
Like many units of land area, the acre was first thought of as a piece of land having certain dimensions. An acre was 40 perches long and 4 perches wide. (The king’s perch was 5½ yards). A strip 40 perches long and 1 perch wide was a rood (not to be confused with the rod, a name from the Saxon gyrd used by the 13th century for the perch.) Not until much later (the 16th century, according to R. D. Connor) did the acre began to be thought of in geometric terms, as so many square feet or square rods.
The length of the acre, 40 perches, was roughly the distance a team of oxen could plow before needing a breather (this furrow-long became the furlong, 220 yards). Ploughmen prefer long furrows, as turning the team is a cumbersome process.
Regarding the width of the acre, 4 perches, see definition 4, below.
In actual use the size of the acre varied greatly, generally being larger in poor land than in good. In some contexts it was almost synonymous with “small holding.”
Another complicating factor is that there were a variety of perches:
The king's rod or perch, however, remained constant for centuries at 16½ feet.
1. Council Directive of 20 December 1979 on the approximation of the laws of the Member States relating to units of measurement and on the repeal of Directive 71/354/EEC. On the web at http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CONSLEG:1980L0181:19791221:EN:PDF
2. Proposal for a Directive of the European Parliament and of the Council amending Council Directive 80/181/EEC on the approximation... On the web at http://europa.eu/eur-lex/lex/LexUriServ/LexUriServ.do?uri=COM:2007:0510:FIN:EN:PDF See item 6.
In Scotland, a unit of land area = 4 (Scots) roods = 160 square falls (about 0.509 hectares or 6150.4 square yards).
As late as the 20th century, several other acres were in use:
In medieval England, 11th – 13th century, a unit of length = 4 perches or rods, = 66 feet, the width of the original acre. It usually, if not always, appears as part of a qualifying phrase that indicates the width is meant. For example:
"three acres wide"
#781 in Kemble, Codex Diplomaticus Aevi Saxonici.
"..iii acera bræda..."
Legis Æðelstan, IV 5.
"...ix acrae latitudine..."
Legis Hen I, cap. xvi.
In the 17th century this distance became the length of the surveyor's chain. This length is also the distance between wickets in cricket, and the width of the strip of land that could be acquired by eminent domain for a road in the less-developed parts of the British Commonwealth.
In England the acre was also a unit of tax assessment. As such, it was not strictly related to the actual dimensions of the property. The following terms are mostly used by modern scholars:
In Normandy, 13th – 15th centuries?, a unit of land area = 160 square perches (each perch of 22 pieds), = 77,449 square pieds, about 8172 square meters. This unit was probably brought back from England to Normandy in the years following the Norman invasion. Local variations were plentiful, varying with the length of the perch.
H. Navel (Commandant).
Recherches sur les anciennes mesures agraires normandes. Acres, vergées
et perches.
Caen: Jouan et Bigot, 1932.
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Last revised: 3 February 2006.