What is an Acre ?
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.
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, 4 by 40, to accommodate ploughing
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 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, because turning the team is a cumbersome process.
A strip 40 perches long and 1 perch wide was called a rood (not to be confused with the rod , a name from the Saxon gyrd used by the 13th century as a synonym for the perch.) So an acre was 4 roods. Not until much later (the 16th century , according to R. D. Connor) did most people begin to think of the acre as so many square feet or square rods.
In actual use in the Middle Ages 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. As you can see, the area of the acre depends on the length of the perch.
The king's rod or perch, however, remained constant for eight centuries at 16½ feet, and that perch set the size of the statute acre.
A Common Land Unit (CLU) is the smallest unit of land that has a permanent, contiguous boundary, a common land cover and land management, a common owner and a common producer in agricultural land associated with USDA farm programs. CLU boundaries are delineated from relatively permanent features such as fence lines, roads, and/or waterways.
The (statute) acre is:
•= 43,560 square feet
•= 4,840 square yards
•= 160 square rods
•= 1/640 square mile (that is, 0.001 562 5 sq. mile)
•about 0.404 687 3 hectare
(Hectare – Metric unit of area equal to 10,000 square meters, or 2.471 acres, or 107,639 square feet.)
Since medieval times the acre hasn't had any specific dimensions. It is purely an area of 43,560 square feet. The two sides of a 1-acre rectangular lot can be any lengths as long as multiplying one by the other gives 43,560 (if they are measured in feet). For example, imagine a sidewalk 5 feet wide. If it were (43560 ÷ 5 = ) 8712 feet long it would take up an acre, a long skinny acre. On the other hand, if the 1-acre lot were a square, its sides would be only 208.7 feet.
Acre – The English acre is a unit of area equal to 43,560 square feet, 10 square chains, or 160 square poles. It originates from a plowing area that is 4 poles wide and a furlong (40 poles) long. A square mile is 640 acres. A Scottish acre is 1.27 English acres. An Irish acre is 1.6 English acres.
Pole – Unit of length and area. Also known as a perch or rod. As a unit of length, one pole is equal to 16.5 feet. A mile is 320 poles. As a unit of area, one pole is equal to a square with sides one pole long. An acre is 160 square poles. It was common to see an area referred to as “87 acres, 112 poles”, meaning 87 and 112/160 acres.
U.S. Survey foot :
one foot = 1200/3937 meter.
This foot is now known as the U.S. Survey foot, = 1.000 002 international feet, and is used only for land measurements.