Feet to Inches (ft to in)

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).

One international inch is exactly 25.4 millimetres — equivalently 0.0254 metre, or precisely one-twelfth of an international foot. The defining factor is conventionally written against the millimetre rather than the metre because 25.4 is itself a finite decimal: the inch is an exact rational submultiple of the SI metre with no measurement uncertainty in the conversion at all. The standard textual symbol is "in"; in mechanical drawings, architectural plans and machinist's notation the inch is written with the double prime ″ (Unicode U+2033), paired with the single prime ′ (U+2032) for the foot, so a height of six feet two inches is correctly rendered 6′ 2″. Subdivisions follow two parallel conventions. Traditional carpentry, dressmaking and consumer rulers halve the inch repeatedly into 1/2, 1/4, 1/8, 1/16, 1/32 and 1/64 — the fractional inch. Machine-shop and engineering practice instead works in decimal inches: a 0.250-inch drill bit and a 1/4-inch drill bit name the same hole, but the decimal form composes more cleanly with calliper readings and CNC G-code. Below the decimal inch sits the thousandth — the thou (British) or mil (American), at exactly 0.0254 mm or 25.4 micrometres — the standard tolerance unit on Anglo-American machine drawings throughout the twentieth century.

The inch is the smallest unit in everyday English measurement that survived from the Roman duodecimal system into modern legal use, and it carries the etymology of that lineage in plain sight: "inch" descends from the Latin uncia, meaning "one-twelfth", the same Roman fraction that gave the troy pound its twelve subdivisions and the Roman foot its twelve unciae. The medieval English inch was rationalised informally against the human body — a thumb-width was the most common reference, a connection still legible in the French pouce and the Italian pollice, both of which mean "thumb" as well as "inch". Legally fixed definitions arrived earlier than for the foot above it: a statute conventionally attributed to Edward II around 1324 defined the inch as the length of three round and dry grains of barley laid end to end, an attempt at a reproducible standard that any market trader could verify with a handful of grain. By the Tudor era the inch was firmly anchored as one-twelfth of the statute foot, with thirty-six inches to the yard formalised in Elizabeth I's measurement statutes. The decimal-friendly modern value, however, emerged from twentieth-century manufacturing rather than from royal proclamation. Industrial standards bodies fixed it before any diplomatic instrument did: the British Standards Institution adopted an "industrial inch" of exactly 25.4 millimetres in 1930, and the American Standards Association followed in 1933, settling the value that screw threads, machine tools, gauge blocks and aircraft fasteners on both sides of the Atlantic would share. Those parallel industrial conventions anticipated the legal harmonisation by more than a quarter of a century, so by the time the inch was internationally agreed for customary trade use its modern value was already a long-settled engineering fact.

The inch survives far more vigorously than its parent foot in industrial, design and digital contexts where the foot would be too coarse. Display sizes for televisions, computer monitors, laptops, tablets and smartphones are quoted globally in diagonal inches regardless of the surrounding metric environment — Apple's iPhone 15 6.1-inch panel and Samsung's 65-inch QLED are spec'd in inches in Tokyo, Berlin, São Paulo and Mumbai. Pixel densities use the same denomination: PPI (pixels per inch) for emissive displays and DPI (dots per inch) for printers and scanners are cited per inch even in publishing workflows that are otherwise fully metric. CSS itself bakes the inch into the web platform: the W3C CSS Values and Units Module Level 3 specification fixes 1in to exactly 96px as a CSS-internal equivalence, with the px itself defined as a reference pixel calibrated against the visual angle of 1/96 inch on a 96-dpi screen at arm's length, rather than as a literal 1/96 inch of physical glass on every device. Inside the layout engine 1in, 96px and 25.4mm resolve to the same value, but the rendered block size on the user's actual display still depends on the medium, the viewport meta tag, zoom and devicePixelRatio. Camera sensors carry an older inheritance — the 1950s vidicon-tube nomenclature in which a "1-inch type" sensor refers to the historical tube outer diameter, not the imaging area, so a 1-inch type sensor today measures roughly 13.2 × 8.8 mm. Wheel-rim sizes for cars, motorcycles and bicycles are denominated in inches everywhere, sitting awkwardly inside European tyre specifications that quote section width and aspect ratio in millimetres and percent. American plumbing runs on half-inch supply lines and three-quarter-inch mains; PCB and chip-package design still spec trace widths and pad pitches in mils; firearms label bore diameter in the .22, .38 and .45 calibre conventions; and the vinyl record industry never dropped its 7-, 10- and 12-inch formats.