Foot (unit)
The foot (standard symbol: ft) is a unit of length in the British imperial and United States customary systems of measurement. The prime symbol, ′, is commonly used to represent the foot. In both customary and imperial units, one foot comprises 12 inches, and one yard comprises three feet. The United States is the only industrialized country that uses the international foot in preference to the meter in many commercial, engineering, and standards activities. The foot is legally recognized in the United Kingdom and as an alternative expression of length in Canada. The measurement of altitude in international aviation is one of the few areas where the foot is used outside the English-speaking world.
- Type
- Unit of length
- Systems
- British imperial, US customary
- Symbol
- ft, ′
- Subunits
- 12 inches per foot, 3 feet per yard
- Notable usage
- Altitude in international aviation
Lore & Background
Historically, the foot was part of many local systems, including Greek, Roman, Chinese, French, and English systems. A 16th-century procedure by Jacob Köbel described verifying the foot by having 16 men leaving church place their left feet one behind the other, with one sixteenth of that length being the lawful foot.
Reader's Guide
The foot remains a significant unit of length in the United States for many commercial, engineering, and standards activities, and is legally recognized in the United Kingdom and Canada. Its definition was standardized internationally in 1959, ending centuries of variation. The foot's historical importance is evident in its use across multiple ancient civilizations, including Greek, Roman, Chinese, and English systems, where it was often subdivided into 12 inches or 16 digits. The foot's persistence in aviation altitude measurement demonstrates its continued relevance beyond the English-speaking world. The transition from diverse local definitions to a single international standard reflects broader efforts toward measurement harmonization, though the United States remains the only industrialized country to prefer the foot over the meter in many contexts.
Did You Know?
- A 16th-century method for verifying the foot involved 16 men placing their left feet one behind the other after church.
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