A foot (plural: feet or foot; symbol or abbreviation: ft or, sometimes, ′ – a prime) is a unit of length, in a number of different systems, including English units, Imperial units, and United States customary units. Its size can vary from system to system, but in each is around a quarter to a third of a metre. The most commonly used foot today is the international foot. There are 3 feet in a yard and 12 inches in a foot.
In 1958 the United States and countries of the Commonwealth of Nations defined the length of the international yard to be 0.9144 metres. Consequently, the international foot is defined to be equal to 0.3048 metres (equivalent to 304.8 millimetres).
The international standard symbol for a foot is "ft" (see ISO 31-1, Annex A). In some cases, the foot is denoted by a prime, which is often approximated by an apostrophe, and the inch by a double prime. For example, 5 feet 2 inches is denoted by 5′2″. This use can cause confusion, because the prime and double prime are also international standard symbols for arcminutes and arcseconds. The symbol ft is shared with that for the femtotonne.
United States survey foot
The United States survey foot is defined as exactly 1200⁄3937 metres, approximately 0.30480061 m. It is used in connection with surveying and mapping. It is exactly 1,000,000 / 999,998 times the international foot and thus 610 nm greater than the international foot. 
The small difference between the survey and international feet would not be detectable on a survey of a small parcel, but becomes significant for mapping, or when a state plane coordinate system is used, because the origin of the system may be hundreds of kilometers (miles) from the point of interest. In 1986 the National Geodetic Survey (NGS) released the North American Datum of 1983, which underlies the state plane coordinate systems. An NGS policy from 1991 has this to say about the units used with the new datum:
In preparation for the adjustment of the North American Datum of 1983, 31 states enacted legislation for the State Plane Coordinate System of 1983 (SPCS 83). All states defined SPCS 83 with metric parameters. Within the legislation, the U.S. Survey Foot was specified in 11 states and the International Foot was specified in 6 states. In all other states the meter is the only referenced unit of measure in the SPCS 83 legislation. The remaining 19 states do not yet have any legislation concerning SPCS 83.
The foot as a measure was used in almost all western cultures and was usually divided into 12, sometimes 10 inches/thumbs or into 16 fingers/digits. The first known standard foot measure was from Sumer, where a definition is given in a statue of Gudea of Lagash from around 2575 BC. Some metrologists speculate that the imperial foot was adapted from an Egyptian measure by the Greeks, with a subsequent larger foot being adopted by the Romans.
Some of the earliest records of the use of the foot come from the region of ancient Greece. The originators devised, or perhaps borrowed from Egypt, the degree of longitude, divided the circumference of the earth into 360 degrees, and subdivided the degree for shorter distances. One degree of longitude comprised 600 stadia. One stadion was divided into 600 feet. Thus the degree of longitude measured 360,000 feet. One mile was 10 stadia or 6000 feet. This is essentially the same mile that was (or still is) used in the Western hemisphere, but the modern foot is longer than the original.
The popular belief is that the original standard was the length of a man’s foot. This is most likely true, but when local authorities and national rulers began calibrating and defining measurements, the foot of no human being was probably used as the basis. In rural regions and without calibrated rulers, many units of measurement were in fact based on the length of some part of body of the person measuring (or for example the area that could be ploughed in a day). In that sense, the human foot was no doubt the origin of the measuring unit called a "foot" and was also for a long time the definition of its length. To prevent discord and enable trade, many towns decided on a standard length and displayed this publicly. In order to enable simultaneous use of the different units of length based on different parts of the human body and other "natural" units of length, the different units were redefined as multiples of each other, whereby their lengths no longer corresponded to the original "natural" standards. This process of national standardisation began in Scotland in 1150 and in England in 1303, but many different regional standards had existed in both these countries long before.
Some believe that the original measurement of the English foot was from King Henry I, who had a foot 12 inches long; he wished to standardise the unit of measurement in England. However this is unlikely, because there are records of the word being used approximately 70 years before his birth (Laws Æthelstan). This of course does not exclude the possibility that this old standard was redefined ("calibrated") according to the ruler's foot. In fact, there is evidence that this sort of process was common at least in earlier ages. In other words, a new important ruler could try to impose a new standard for an existent unit, but it is unlikely that any king's foot was ever as long as the modern unit of measurement.
The average foot length is about 9.4 inches (240 mm) for current Europeans. Approximately 99.6% of British men have a foot that is less than 12 inches long. One attempt to "explain" the "missing" inches is that the measure did not refer to a naked foot, but to the length of footwear, which could theoretically add an inch or two to the naked foot's length. This is consistent with the measure being convenient for practical uses such as building sites. People almost always pace out lengths whilst wearing shoes or boots, rather than removing them and pacing barefoot.
There are however historical records of definitions of the inch based on the width (not length) of a man's thumb that are very precise for the standards of the time. One of these was based on an average calculated using three men of different size, thereby enabling surprising accuracy and uniformity throughout a country even without calibrated rulers. It therefore seems likely that at least since about the 12th century the precise length of a foot was in fact based on the inch, not the other way around. Since this length was fairly close to the size of most feet, at least in shoes, this enabled the above-mentioned use of one's shoes in approximating lengths without measuring devices. This sort of imprecise measuring that in addition excessively multiplied the measuring error due to repeated use of a short "ruler" (the foot) was of course never used in surveying and in constructing more complicated buildings.
- Units of measurement
- History of measurement
- Systems of measurement
- Weights and measures
- English unit
- Imperial unit
- United States customary units
- International System of Units
- BBC World Service
- A. V. Astin & H. Arnold Karo, (1959), Refinement of values for the yard and the pound, Washington DC: National Bureau of Standards, republished on National Geodetic Survey web site and the Federal Register (Doc. 59-5442, Filed, June 30, 1959, 8:45 a.m.)
- National Geodetic Survey, (January 1991), "POLICY OF THE NATIONAL GEODETIC SURVEY CONCERNING UNITS OF MEASURE FOR THE STATE PLANE COORDINATE SYSTEM OF 1983.
ast:Pie (unidá) ay:Kayu takt'a bs:Stopa bg:Фут cs:Stopa (jednotka délky) da:Fod (længdeenhed) de:Fuß (Einheit) dv:ފޫޓު et:Jalg (pikkusühik) eo:Futo eu:Oin (unitatea) gl:Pé (medida) ko:피트 hr:Stopa id:Kaki (satuan panjang) it:Piede (unità di misura) he:רגל (יחידת מידה) ka:ფუტი sw:Futi hu:Láb (mértékegység) mk:Стапка (мерка) nl:Voet (lengtemaat) no:Fot (mål) nn:Eininga fot uz:Fut simple:Foot (unit of length) sl:Čevelj (dolžinska mera) sr:Стопа (јединица) sh:Stopa (jedinica za dužinu) fi:Jalka (mittayksikkö) sv:Fot (enhet) ta:அடி th:ฟุต uk:Фут wa:Pî (unité)