Meters to Feet (m to ft)

The metre (m) is defined as the length of the path travelled by light in a vacuum during a time interval of 1/299,792,458 of a second. The definition fixes the speed of light c at exactly 299,792,458 m/s, making c a defined constant rather than a measured quantity since 1983; the second on which it depends is itself defined by the unperturbed ground-state hyperfine transition of the caesium-133 atom. National metrology institutes realise the metre with stabilised lasers whose vacuum wavelengths are recommended in the BIPM's mise en pratique — most commonly an iodine-stabilised helium-neon laser at 633 nm, a methane-stabilised helium-neon laser at 3.39 µm, or, more recently, optical frequency combs that link any laser frequency directly to the caesium standard. The metre is the SI base unit of length and the parent of the standard SI prefixes for length: 1 km = 10³ m, 1 cm = 10⁻² m, 1 mm = 10⁻³ m, 1 µm = 10⁻⁶ m, 1 nm = 10⁻⁹ m, and so on down to the femtometre used in nuclear physics.

The meter takes its name from the Greek metron, meaning "measure", and traces its modern existence to the rationalising impulse of the late eighteenth century. An earlier proposal by Christiaan Huygens in 1675 to anchor the unit to a seconds pendulum — a pendulum whose half-period equals one second, which on Earth happens to be very close to a metre long — was eventually rejected because the local strength of gravity varies with latitude, so a pendulum-defined length would differ measurably between Paris and Quito. In 1791 a commission of the French Academy of Sciences (Borda, Lagrange, Laplace, Monge and Condorcet) proposed instead that the metre be one ten-millionth of the distance from the North Pole to the equator measured along the meridian passing through Paris. From 1792 to 1799, surveyors Jean-Baptiste Delambre and Pierre Méchain measured a section of that meridian arc from Dunkirk to Barcelona — a seven-year geodetic project carried out under the chaos of revolutionary and Napoleonic Europe, and famously troubled by inconsistencies in Méchain's Barcelona latitude observations that he concealed and never reconciled. The resulting platinum end-bar, the Mètre des Archives, was deposited in the French National Archives on 22 June 1799 and became the first physical metre; later geodesy showed the bar to be about 0.2 mm shorter than one ten-millionth of the actual meridian quadrant, principally because the calculation assumed a value of Earth's flattening that did not match reality. The Convention of the Metre, signed in Paris on 20 May 1875 by seventeen nations, created the BIPM and put the unit under international stewardship. At the 1st CGPM in 1889 a new International Prototype Metre — a 90% platinum, 10% iridium bar with the X-shaped Tresca cross-section — replaced the Archives bar, and thirty witness copies were distributed by lottery to signatory states. The 11th CGPM in 1960 abandoned the artefact altogether, redefining the metre as exactly 1,650,763.73 wavelengths of the orange-red emission line of krypton-86. Then the 17th CGPM, on 21 October 1983, fixed the speed of light at exactly 299,792,458 m/s and tied the metre to it — the definition that survives, in slightly reframed form, after the 2019 SI overhaul.

The metre is the legal unit of length in nearly every country on Earth — the United States, Liberia and Myanmar are the conventional shorthand for the three states that have not formally adopted SI for everyday commerce, although in practice all three use the metre extensively in science, medicine, the military and trade with metric partners. Across the European Union it is mandatory for trade, labelling and engineering specification under directive 80/181/EEC. The United Kingdom completed its statutory metrication of trade in 2000 but retains miles and yards on road signs and the imperial pint for draught beer; British architects, surveyors, builders and scientists work entirely in metres and millimetres. American science, engineering, medicine, pharmaceuticals and the entire NATO military supply chain use the metre, while everyday measurements of height, room dimensions and road distance stay in feet, yards and miles. Track-and-field athletics is metric worldwide — the 100-metre dash, the marathon at 42.195 km, all field events — as is competition swimming (in 25 m or 50 m pools), association football (pitches and goal dimensions), basketball, and Olympic gymnastics. Cinema and photography retain a metric inheritance in film widths (35 mm, 16 mm, 8 mm) and lens focal lengths. Aviation is the salient exception: ICAO conventions still record vertical altitude in feet and horizontal distance in nautical miles, even in fully metric jurisdictions, because air-traffic-control phraseology has never been re-standardised.

One international foot equals exactly 0.3048 metre by the International Yard and Pound Agreement of 1959. The foot is divided into 12 inches; three feet make one yard, and 5,280 feet make one statute mile. The standard textual symbol is "ft"; in technical drawings, architectural plans and surveying notation the prime symbol ′ (Unicode U+2032) is used for the foot and the double prime ″ (U+2033) for the inch, so a height of six feet two inches is correctly written 6′ 2″. The earlier US Survey Foot, defined by the 1893 Mendenhall Order as 1200/3937 metre (approximately 0.304800609 m) and retained for geodetic and state-plane work long after 1959, was jointly deprecated by NIST and NOAA's National Geodetic Survey effective 1 January 2023. The foot is recognised by NIST for customary use under Federal Register notice 24 FR 5445, the same instrument that recognises the avoirdupois pound.

The English foot belongs to a family of body-derived length units that descend, by way of the Roman pes, from the Greek pous: the Olympic foot used in stadium-track measurement was about 316 mm and the shorter Attic foot about 308 mm, both rooted in the practical idea that a useful unit of length should be roughly the length of an adult man's foot. The Roman pes, around 296 mm, was divided into sixteen digiti or twelve unciae — and that twelve-part division is the direct ancestor of the twelve-inch foot we use today. Medieval Europe again produced regional variants: the Belgic or "northern" foot at roughly 333 mm, the rather shorter North German foot at about 285 mm, and a clutch of English county and trade feet that varied measurably from one market town to the next. Standardisation in England was a long, statutory affair. Edward I's Compositio Ulnarum et Perticarum, around 1303, fixed three feet to one yard, and Elizabeth I's statute of 1593 set the statute mile at 5,280 feet — eight furlongs of 660 feet apiece. The Imperial Weights and Measures Act of 1824 consolidated those statutes around a brass yard standard kept at Westminster, which was destroyed alongside the pound and yard standards in the 1834 Palace of Westminster fire. The defining moment of the modern foot, however, was the International Yard and Pound Agreement, signed on 1 July 1959 by the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa: it set the international foot at exactly 0.3048 metre and so closed a small but persistent transatlantic gap between the American and Commonwealth definitions. The American foot had been defined since the 1893 Mendenhall Order as 1200/3937 metre — about 0.30480061 m — and that older value, renamed the US Survey Foot, was retained for geodetic surveying and state-plane coordinate systems for another six decades. NIST and NOAA's National Geodetic Survey jointly deprecated the US Survey Foot effective 1 January 2023, after a public-comment process that began in October 2019; from that date the international foot of exactly 0.3048 m is the sole legally-recognised foot in the United States.

Aviation is the foot's most prominent surviving professional habitat. ICAO standards record aircraft altitudes in feet worldwide — a typical commercial cruise of 35,000 ft is universally referred to as "flight level 350" (FL350), regardless of whether the surrounding airspace is metric for any other purpose — and pilot training, autopilot logic, certified altimeters, terrain charts and air-traffic-control phraseology all run on feet. Russia and several post-Soviet states held out for metres until the 2010s, when most aligned with ICAO feet, leaving China and North Korea as the conspicuous remaining holdouts for metric vertical separation in controlled airspace. Outside aviation, the foot remains everyday currency in the United States: residential and commercial construction, real estate listings, dimensional lumber, pipe and conduit lengths, ceiling heights, and personal height all default to feet and inches. The petroleum industry — including national oil companies in fully metric jurisdictions — measures drilling depth in feet, a convention inherited from American oilfield equipment and tool joints. The United Kingdom officially metricated for trade in 2000 but still posts low-bridge and tunnel-clearance signs in feet and inches alongside metres, and personal height is still quoted in feet and inches across casual British, Irish and Canadian usage. American football and Canadian football mark out the field in yards and feet (the 100-yard playing field plus two 10-yard end zones is 360 ft from end-line to end-line).