Foot (unit)

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See also: U.S. customary units

The foot (plural foot or feet; symbol or abbreviation: ft or the single prime ′ ) is a unit of length in the Imperial and U.S. customary systems of measurement. It is equal to 12 inches or one third of a yard.

It is used primarily in the United States of America and, although it has been officially superseded by units of the metric system, it is still commonly used in the United Kingdom .

The human foot has been widely used as a unit of measurement throughout history. The natural foot (Latin pes naturalis), an ancient unit based directly on the length of human feet, was about 25 centimetres (cm) (about 10 inches) long.

This unit was replaced in early Middle Eastern civilizations by a longer foot, roughly the length of the modern unit, that was an exact multiple of other natural units: 1 foot = 3 hands = 4 palms = 12 inches (thumb widths) = 16 digits (finger widths). This unit was used in both Greece and Rome; the Greek foot is estimated at 30.8 cm (12.1 inches) and the Roman foot (pes) at 29.6 cm (11.7 inches). The latter was divided into 12 unciae (inches).

A manual foot (Latin pes manualis) of about 33.3 cm (13.1 inches) was more common in northern Europe. The manual foot was equal to two shaftments, each the width of the fist and outstretched thumb. A man could easily measure a shaft, rod or such like by hand, gripping it with both hands, thumbs extended and touching, and moving the grip hand over hand the whole length of the shaft.

In England, the Roman foot was replaced after the fall of Rome by the natural foot and the shaftment. The modern foot (of about 30.5 cm) did not appear until after the Norman Conquest of 1066. It may have been an innovation of Henry I. Later in the 1100s, a "standard" foot was inscribed on the base of a column of St. Paul's Church in London. Many other countries have had units of similar lengths, each called by the local word for the human foot: German fuß, French pied, Hungarian láb, and so on.

The 1875 Convention du Mètre established three organizations to oversee the keeping of metric standards. The U.S. signed this treaty in 1876 and the UK in 1884; subsequently, the foot was officially defined in terms of the metre. In the U.S., Bulletin 26, "Fundamental Standards of Length and Mass", of the U.S. Coast and Geodetic Survey, approved for publication April 5, 1893, defined the foot as exactly 1200/3937 metre, or about 30.480 060 96 cm. This unit, now called the U.S. Survey Foot, or informally just the survey foot, is still used for geodetic surveying in the U.S. and in some legal documents.

In 1959, the U.S. National Bureau of Standards redefined the foot as exactly 30.48 cm (about 0.999 998 survey foot). This definition was also adopted in the UK by the Weights and Measures Act of 1963. Thus, the foot of 30.48 cm is now sometimes called the international foot.