A foot (plural: feet) is a non-SI unit of distance or length. The popular belief is that original standard was the length of a man's foot. The average foot length is about 240 mm (9.4 in) for current Europeans. About 996 of 1000 British men have a foot that is less than 12 inches long. A plausible explanation for the missing inches is that the measure did not refer to a naked foot, but to the length of footwear. This is consistent with the measure being convenient for practical purposes such as on building sites etc. People almost always pace out lengths whilst wearing shoes or boots, rather than removing them and pacing barefoot.
There are twelve inches in one foot and three feet in one yard.
The standardization of weights and measures has left several different standardized foot measures. The most commonly used foot today is the imperial foot, which is defined to be exactly 0.3048 metres. This unit is sometimes denoted with a prime (e.g. 30′ means 30 feet), often approximated by an apostrophe. Similarly, inches can be denoted by a double prime (often approximated by a quotation mark), so 6′2″ means 6 feet 2 inches.
The foot as a measure was used in almost all cultures. The first known standard foot measure was from Sumeria, where a definition is given in a statue of Gudea of Lagash from around 2575 BC. The imperial foot was adapted from an Egyptian measure by the Greeks, with a subsequent larger foot being adopted by the Romans.
In addition to the current standard imperial foot, there is also a slightly different U.S. survey foot, used only in connection with surveys by the U.S. Coast and Geodetic Survey, defined as exactly 1200/3937 m. (http://www.ngs.noaa.gov/PUBS_LIB/FedRegister/FRdoc59-5442.pdf)
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In metric countries the unit corresponding to the acre is the hectare, which is 10,000 square meters (the area of a square 100 meters on each side).
The X unit and the angstrom star are the units used for this purpose.
One astronomical unit equals the "average" distance from the center of the Earth to the center of the Sun (mathematically, it is the length of the semimajor axis of the Earth's elliptical orbit, which is the ordinary average of the Earth's minimum and maximum distances from the Sun).
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