¿Qué es un/una Foot (ft)?
Formal Definition
The foot (symbol: ft, plural: feet) is a unit of length in the imperial and US customary measurement systems. Since the international yard and pound agreement of 1959, one foot is defined as exactly 0.3048 meters, or exactly 30.48 centimeters. One foot equals 12 inches, and 3 feet equal one yard.
Practical Scale
The foot corresponds roughly to the length of an adult human foot, which is its historical origin. A standard ruler is one foot long. A standard sheet of US letter paper is approximately one foot long (11 inches). The foot provides a human-scale measurement unit that is natural and intuitive for describing heights, room dimensions, and moderate distances.
Relationship to Other Units
The foot sits between the inch and the yard in the imperial system: 12 inches make a foot, and 3 feet make a yard. There are 5,280 feet in a mile. In the US customary system, the foot (not the yard or meter) is the primary unit for expressing human height, building dimensions, and elevation. In aviation, the foot is the worldwide standard for measuring altitude.
Etymology
Body-Based Measurement
The word "foot" derives from the Old English "fot," which meant both the body part and the unit of length. The use of the foot as a unit of measurement is among the oldest known, appearing in virtually every ancient civilization. The connection between the body part and the measurement unit exists in most European languages: Latin "pes," French "pied," German "Fuß," Spanish "pie," and Russian "фут."
Ancient Precedents
The foot as a measurement unit predates written history. Ancient Egyptian, Mesopotamian, Greek, and Roman civilizations all used foot-length units, though their exact lengths varied. The Roman foot (pes) measured about 29.6 cm, slightly shorter than the modern foot. The Greek foot was about 30.8 cm. These variations reflected different local standards, often based on the foot size of a ruler or standard-setter.
Standardization in England
The English foot was gradually standardized over centuries. The Composition of Yards and Perches (circa 1300) defined the foot as 12 inches, with an inch being the length of three barleycorns laid end to end. While this definition was imprecise, the 12-inch foot became firmly established in English law and commerce, eventually evolving into the precisely defined unit used today.
Precise Definition
International Foot
Since 1 July 1959, the international foot has been defined as exactly 0.3048 meters. This makes one foot exactly 304.8 millimeters. The definition was agreed upon by the national standards organizations of the United States, United Kingdom, Canada, Australia, New Zealand, and South Africa to resolve slight discrepancies between their national foot standards.
US Survey Foot
The United States also maintained a slightly different "US survey foot" defined as exactly 1200/3937 meters (approximately 0.3048006096 m), which differed from the international foot by about 2 parts per million. The US survey foot was used for geodetic surveying and land measurement. On 1 January 2023, the US survey foot was officially retired, and the international foot became the sole legal definition of the foot in the United States.
Aviation Standard
The foot is the worldwide standard for measuring altitude in aviation, as established by the International Civil Aviation Organization (ICAO). Aircraft altitudes, cloud ceilings, visibility layers, and terrain elevations are all expressed in feet globally, regardless of whether a country otherwise uses the metric system. This convention exists because the United States and United Kingdom dominated early aviation development and their standards became international norms.
Historia
Ancient Origins
The foot is one of the oldest units of measurement, with evidence of foot-based measurements dating back to at least 3000 BC in Egypt and Mesopotamia. The ancient Egyptian royal cubit (about 52.4 cm) was divided into 7 palms of 4 digits each, but foot-length units were also used. The Roman foot (pes, about 29.6 cm) was widely used across the Roman Empire and influenced measurement systems throughout Europe.
Medieval England
After the fall of Rome, foot measurements in England evolved independently. Various regional feet coexisted, causing confusion in trade. The Statute for Measuring Land (1305) attempted standardization, and by the Tudor period, the 12-inch foot was well established. Henry VII and Elizabeth I both commissioned standard measures that included the foot.
The Imperial System
The Weights and Measures Act of 1824 incorporated the foot into the British Imperial System, defined as one-third of the Imperial Standard Yard. The physical standard yard bar defined both the yard and, by extension, the foot. When the original standard was destroyed in the 1834 Parliament fire, new standards were carefully reconstructed.
Metric Definition
The Mendenhall Order of 1893 defined the US foot in terms of the meter (1 foot = 1200/3937 m). This was the first time the foot was officially tied to the metric system. The 1959 international agreement simplified this to exactly 0.3048 m, creating the international foot used today.
Modern Status
The foot remains one of the most widely used non-metric units in the world, primarily because of its continued use in American daily life and its universal role in aviation. The US is the only major country where the foot is the primary unit for everyday length measurement, but aviation's global use of feet for altitude means that every pilot and air traffic controller worldwide works with feet daily.
Uso actual
In the United States
The foot is the primary unit of length in American daily life. Human height is expressed in feet and inches ("I'm 5 foot 10"). Room dimensions are given in feet ("a 12 by 15 room"). Building codes specify dimensions in feet and inches. Real estate listings describe homes in square feet. Lumber dimensions use feet for length (an 8-foot 2×4). Construction, carpentry, and home improvement all revolve around the foot.
In Aviation
The foot is the universal unit for altitude in civil aviation worldwide. Flight levels are expressed in hundreds of feet (FL350 = 35,000 feet). Cloud ceilings, visibility, runway lengths, and terrain heights are all reported in feet. Even countries that otherwise use metric units exclusively follow this convention. A pilot in France or Japan reads altitude in feet and communicates it in feet to air traffic control.
In the United Kingdom
The UK uses feet for human height (alongside metric) and for some construction applications. Older Britons routinely describe heights in feet and inches. Road tunnel heights and bridge clearances are posted in feet and inches alongside meters. However, the UK has largely transitioned to metric for official purposes.
In Other Countries
Canada, which officially metricated in the 1970s, still uses feet in real estate (house sizes in square feet), construction, and casual conversation about height. India uses feet for human height in everyday conversation, even though the metric system is official. Many countries in the developing world use a mix of feet and meters depending on context.
Everyday Use
Measuring Height
In the US, human height is almost universally expressed in feet and inches. "Six feet tall" or "five-two" are instantly understood. Doorframes are 6 feet 8 inches standard. Basketball rims are 10 feet high. Ceiling heights in homes are typically 8 or 9 feet. This feet-and-inches system for human height is deeply embedded in American (and to a lesser extent British) culture.
Home and Construction
American homeowners and contractors live in a world of feet. Room sizes are described as "10 by 12" (feet). Fence heights are 4, 6, or 8 feet. Ladder lengths are given in feet. Swimming pools are described by depth in feet. Garden hoses come in 25, 50, or 100-foot lengths. Lumber is sold in 8, 10, 12, and 16-foot lengths.
Real Estate
Property size in the US is described in square feet. A starter home might be 1,200 square feet; a mid-range home 2,000 to 3,000 square feet. Commercial office space is leased by the square foot. Lot sizes for land are sometimes given in square feet for urban lots (a 5,000 square foot lot) or acres for larger parcels.
Scuba Diving
Recreational scuba diving uses feet for depth measurement in the US. Recreational diving limits are typically 60 to 130 feet. Dive tables and computers display depth in feet. The no-decompression limit at 60 feet is about 60 minutes, while at 100 feet it is about 20 minutes.
In Science & Industry
Aviation Science
The foot's most significant scientific application is in aviation and atmospheric science. The standard atmosphere model defines pressure altitude in feet, and all aeronautical charts worldwide use feet for elevation. Aviation weather reports (METARs) express cloud heights and visibility in feet. This creates an unusual situation where even metric countries use an imperial unit for critical safety-related measurements.
Fluid Dynamics
In some branches of US engineering, the foot-pound-second (FPS) system is still used. Fluid flow rates may be expressed in cubic feet per second (cfs) or cubic feet per minute (cfm). The US Geological Survey measures river discharge in cubic feet per second. HVAC engineers in the US specify airflow in cubic feet per minute.
Geoscience
In US geological surveys, elevation data has traditionally been recorded in feet. The National Geodetic Survey maintains elevation benchmarks in feet across the United States. Oil and gas well depths are measured in feet. Mining depths are expressed in feet. While the scientific community uses meters, much practical geoscience data in the US remains in feet.
Historical Note
Before the universal adoption of SI units in science, many important measurements were made in feet. The speed of sound was commonly expressed as 1,125 feet per second (at sea level, 59°F). Atmospheric pressure was described using feet of water or inches of mercury. Converting historical scientific data from feet to meters remains an ongoing task in many fields.
Interesting Facts
Aircraft altitude is measured in feet worldwide by international agreement (ICAO). Even pilots in fully metric countries like France, Japan, and Brazil report altitude in feet. Only China and a few former Soviet states have historically used meters for aviation altitude.
The Roman foot (pes) was about 29.6 cm — roughly 3% shorter than the modern foot of 30.48 cm. Roman roads and buildings were laid out using this unit, and many can still be measured to confirm the Roman foot's dimensions.
The US survey foot was exactly 2 millionths of a foot (0.6 micrometers) longer than the international foot. Over the distance of a mile, this difference amounts to about 3.2 millimeters. The survey foot was retired on 1 January 2023.
Mount Everest's height is 29,032 feet (8,849 meters), a figure revised upward by 2.3 feet in a joint survey by Nepal and China in 2020. The height was first measured in 1856 at 29,002 feet by the Great Trigonometrical Survey of India.
A football field (American) is 300 feet (100 yards) long. A soccer/football pitch is between 295 and 394 feet (90-120 meters) long, depending on the venue.
The Burj Khalifa, the world's tallest building, stands 2,717 feet (828 meters) tall. The Eiffel Tower is 1,083 feet (330 meters), and the Empire State Building is 1,454 feet (443 meters) including its antenna.
The speed of sound at sea level (at 68°F/20°C) is approximately 1,125 feet per second. This means sound travels one foot in about 0.89 milliseconds — roughly one foot per millisecond is a useful approximation.
The deepest swimming pool in the world, Deep Dive Dubai, is 196 feet (60 meters) deep. Standard Olympic swimming pools are approximately 6.5 feet (2 meters) deep.
Regional Variations
The United States
The foot is the dominant unit of everyday length measurement in the US. Americans think, speak, and build in feet. This is unlikely to change in the foreseeable future, as feet are deeply embedded in construction codes, real estate practices, and cultural norms.
The United Kingdom
The UK uses a mix of feet and meters. Height is commonly expressed in feet and inches ("I'm 5 foot 8"), but younger Britons often know their height in centimeters as well. Construction has largely moved to metric, but older buildings are documented in feet and inches. Bridge heights on roads show both feet/inches and meters.
Aviation Worldwide
The foot is used for altitude measurement in almost all countries. The notable exceptions are China and some former Soviet states, which have historically used meters in aviation. However, international flights in and out of these countries typically use feet for altitude when communicating with international air traffic control.
South and Southeast Asia
In India, Pakistan, Bangladesh, and several Southeast Asian countries, the foot remains in common everyday use for measuring height and building dimensions, even though these countries officially use the metric system. Real estate in India is commonly described in square feet.