Foot (unit)

The foot (pl. feet; abbreviation: ft; symbol: ′, the prime symbol) is a unit of length in the imperial and US customary systems of measurement. Since the International Yard and Pound Agreement of 1959, one foot is defined as 0.3048 meter exactly. In customary and imperial units, the foot comprises 12 inches and three feet compose a yard. Historically the 'foot' was a part of many local systems of units, including the Greek, Roman, Chinese, French, and English systems. It varied in length from country to country, from city to city, and sometimes from trade to trade. Its length was usually between 250 mm and 335 mm and was generally, but not always, subdivided into 12 inches or 16 digits. The United States is the only industrialized nation that uses the international foot and the survey foot (a customary unit of length) in preference to the meter in its commercial, engineering, and standards activities. The foot is legally recognized in the United Kingdom; road signs must use imperial units (however, distances on road signs are always marked in miles or yards, not feet), while its usage is widespread among the British public as a measurement of height. The foot is recognized as an alternative expression of length in Canada officially defined as a unit derived from the meter although both the U.K. and Canada have partially metricated their units of measurement. The measurement of altitude in international aviation is one of the few areas where the foot is used outside the English-speaking world. The length of the international foot corresponds to a human foot with shoe size of 13 (UK), 14 (US male), 15.5 (US female) or 46 (EU sizing). Historically the human body has been used to provide the basis for units of length. The foot of a white male is typically about 15.3% of his height, giving a person of 160 centimetres (5 ft 3 in) a foot of 245 millimetres (9.6 in) and one of 180 centimetres (5 ft 11 in) a foot of 275 millimetres (10.8 in). Archaeologists believe that the Egyptians, Ancient Indians and Mesopotamians preferred the cubit while the Romans and the Greeks preferred the foot. Under the Harappan linear measures, Indus cities during the Bronze Age used a foot of 13.2 inches (340 mm) and a cubit of 20.8 inches (530 mm). The Egyptian equivalent of the foot—a measure of four palms or 16 digits—was known as the djeser and has been reconstructed as about 30 cm (12 in). The Greek foot (πούς, pous) had a length of ​1⁄600 of a stadion, one stadion being about 181.2 m, therefore a foot being at the time about 302 mm. Its exact size varied from city to city and could range between 270 mm and 350 mm, but lengths used for temple construction appear to have been about 295 mm to 325 mm, the former being close to the size of the Roman foot. The standard Roman foot (pes) was normally about 295.7 mm (97% of today's measurement), but in the provinces, the pes Drusianus (foot of Nero Claudius Drusus) was used, with a length of about 334 mm. (In reality, this foot predated Drusus.)