Kilograms to Ounces (kg to oz)

Since 20 May 2019 the kilogram (kg) is defined by fixing the numerical value of the Planck constant h at exactly 6.62607015 × 10⁻³⁴ when expressed in J·s, which is equivalent to kg·m²·s⁻¹. Because the metre and second appearing in that expression are themselves anchored to the speed of light c and the caesium-133 hyperfine transition frequency Δν_Cs, the kilogram ultimately rides on three fixed constants of nature and can be realised in any sufficiently equipped laboratory without reference to a physical artefact. National metrology institutes do so by one of two routes: a Kibble balance (renamed in 2016 in honour of the late NPL physicist Bryan Kibble, having previously been called the watt balance), which equates electrical and mechanical power to relate mass to the Planck constant via a precisely-measured electromagnetic force; or the X-ray crystal density method, which counts the atoms in a near-perfect spherical single crystal of silicon-28 enriched to roughly 99.995% purity. By international convention the kilogram is the only base unit defined with a prefix in its name, and decimal multiples are formed from the root "gram" rather than "kilogram" — so one million grams is a megagram, not a "kilokilogram".

The kilogram is unique among the seven SI base units in carrying a metric prefix in its very name — a relic of its eighteenth-century origins, when the gramme was defined first and the unit a thousand times larger happened to be the convenient size for everyday weighing. The original legal definition came in the Loi du 18 germinal an III (7 April 1795), the metric law passed during the French Revolution, which fixed the gramme as the mass of one cubic centimetre of water at the temperature of melting ice; the kilogramme was simply its thousand-fold multiple. To realise that abstract definition the French Academy of Sciences commissioned a platinum cylinder, the Kilogramme des Archives, completed in 1799 and held in the National Archives in Paris. The unit's role moved onto the international stage with the Convention of the Metre in 1875, which established the Bureau International des Poids et Mesures (BIPM) at Sèvres just outside Paris. At the 1st General Conference on Weights and Measures (CGPM) in 1889, a new artefact — the International Prototype of the Kilogram (IPK), informally called Le Grand K — was adopted as the world standard: a cylinder roughly 39 mm in both height and diameter cast from a 90% platinum, 10% iridium alloy, with iridium added because pure platinum had proved too soft for an artefact intended to last centuries. For the next 130 years Le Grand K had no measurement uncertainty, because by definition it was one kilogram. The trouble was that periodic verifications against its sister copies — held in 1889, 1948 and 1989 — showed the official copies and the IPK appearing to drift apart by something on the order of fifty micrograms over a century, with the cause never satisfactorily identified. On 16 November 2018 the 26th CGPM voted at Versailles to redefine the unit, and on 20 May 2019 — World Metrology Day — the new definition came into force, ending a 220-year reliance on a single physical artefact.

The kilogram is the legal unit of mass in nearly every country on Earth, recognised by all signatories of the Convention of the Metre as the standard for trade and metrology. Across the European Union it is mandatory for trade, labelling and scientific work under directive 80/181/EEC. The United Kingdom completed its statutory metrication of trade in 2000, with the well-known carve-outs for draught beer and milk sold in returnable containers (still legal in pints) and for road distance and speed signage (still legal in miles and miles per hour). The United States, never officially metricated for everyday commerce, nonetheless requires kilograms or grams alongside customary units on consumer packaging via FDA labelling rules. Healthcare worldwide runs on kilograms regardless of regional preferences for body weight: patients are charted in kg even in American hospitals, because medication dosing is overwhelmingly expressed in milligrams per kilogram of body mass — a convention so universal in paediatrics that any deviation triggers patient-safety review. Olympic sports use kilograms for weight classes apart from boxing, wrestling and mixed martial arts, which inherited their imperial classes from American and British origins. International freight outside US domestic routes, scientific publishing and global commodity markets all denominate mass in kilograms, with the metric tonne (1,000 kg) standard for bulk goods.

The English word "ounce" refers to four distinct units in 2026, three of mass and one of volume. The avoirdupois ounce (oz) is 1/16 of the avoirdupois pound, equal to exactly 28.349523125 g via the 1959 International Yard and Pound Agreement. This is the ounce of US food packaging, postal rates and everyday goods. The troy ounce (oz t, ozt) is 1/12 of the troy pound, equal to exactly 31.1034768 g — about 9.7% heavier than the avoirdupois ounce — and is the global trading unit for gold, silver, platinum and palladium, with spot prices on every major precious-metals exchange (LBMA, COMEX, Shanghai Gold Exchange, Tokyo Commodity Exchange) quoted in US dollars per troy ounce. The apothecaries' ounce, numerically identical to the troy ounce at 31.1034768 g, was the unit of British pharmacy weight under the 1618 Pharmacopoeia Londinensis system but was abolished from UK pharmacy by the Weights and Measures Act 1976; it survives only in occasional historical references and in some US compounding-pharmacy texts. The fluid ounce is a unit of volume rather than mass, and even within volume splits into the US fluid ounce at 29.5735 mL and the UK imperial fluid ounce at 28.4131 mL — a 4% gap that catches recipe transcription between the two systems. Neither fluid ounce should be confused with the mass ounces despite the shared word.

The ounce takes its name from the Roman uncia, the 1/12 subdivision of the libra (the Roman pound) used as both a mass and a linear measure: one uncia of mass was about 27.3 g, one uncia of length one twelfth of the Roman pes (foot), and the same word served both. The mass and length senses survived the empire as separate units, with the linear uncia becoming the inch in English and the mass uncia becoming the ounce — a divergence whose etymological echo survives in the shared root of the two modern words. Two parallel mass ounces emerged in late-medieval English commerce. The avoirdupois ounce, 1/16 of the avoirdupois pound, became the unit of general goods through the same merchant standardisation that fixed the avoirdupois pound. The troy ounce, named for the great medieval trading fair at Troyes in Champagne, was the unit of precious metals: a lighter pound and a 12-ounce subdivision were inherited from the French fair through Anglo-French commerce, and Henry VIII's Coinage Act of 1527 fixed troy weight as the legal standard for English coinage and bullion, the role it has held in English-speaking precious-metals trade ever since. A third ounce — the apothecaries' ounce, numerically identical to the troy ounce at 31.1034768 g but operating within an entirely separate pharmacy-weight system that subdivided into drachms, scruples and grains — was formalised in the Pharmacopoeia Londinensis published by the Royal College of Physicians in 1618 and remained the legal British pharmacy unit for the next three and a half centuries. The Weights and Measures Act 1976 abolished apothecaries' weight from UK pharmacy, leaving the troy ounce as the surviving 31.1-gram unit. The avoirdupois ounce, meanwhile, was given its modern precise value through the 1959 International Yard and Pound Agreement's fixing of the avoirdupois pound at 0.45359237 kg, making 1 oz exactly 28.349523125 g.

Precious-metals trading is the troy ounce's domain. Spot prices on the London Bullion Market Association (LBMA), the COMEX division of CME Group in New York, the Shanghai Gold Exchange and the Tokyo Commodity Exchange are all quoted in US dollars per troy ounce, and gold and silver bars sold to investors are stamped with their troy-ounce weight rather than any metric figure. The London Platinum and Palladium Market and the LBMA Silver Price likewise denominate in troy ounces. The avoirdupois ounce dominates US everyday-goods commerce. The Federal Trade Commission's Fair Packaging and Labeling Act and FDA labelling rules require net weight on consumer goods in pounds-and-ounces or ounces alone, with metric grams alongside, and US food-packaging weights — the 8-oz brick of cream cheese, the 16-oz peanut butter jar, the 5-oz can of tuna — are everyday avoirdupois figures. USPS first-class letter rates in 2026 step up at 1, 2 and 3.5 ounce thresholds, with the 1-ounce minimum the rate break embedded in nearly all US letter mailings. Boxing gloves are denominated in avoirdupois ounces by every major sanctioning body. The WBC, WBA, IBF and WBO standardise glove weight by class: 8-oz gloves for professional bouts up to 147 lb (welterweight), 10-oz gloves above that, with 12-, 14- and 16-oz gloves used in training and amateur competition. The 8/10-oz divide at welterweight is one of the few places in modern professional sport where the imperial unit is the binding contractual specification rather than a converted figure. US spirits and cocktail measurement uses the fluid ounce: a standard US "shot" is 1.5 fl oz (44.4 mL), and US bar-mix recipes denominate every ingredient in fl oz. Soft-drink cans in the US sell as 12 fl oz (354.9 mL), against the European 330 mL standard — a 25 mL gap per can between US Coca-Cola and the same brand's European equivalent.