Ounces to Grams (oz to g)

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system but was abolished from UK pharmacy by the Weights and Measures Act 1976; it survives only in occasional historical references and in some US compounding-pharmacy texts.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.

The gram (g) is one one-thousandth of the kilogram. By international convention adopted at the 11th CGPM in 1960 and unchanged since, decimal multiples and submultiples of mass are formed from the root "gram" rather than from the base unit "kilogram" — so 1,000 grams is one kilogram, one million grams is a megagram (Mg, equivalently a metric tonne), and one-thousandth of a gram is a milligram (mg), not a "microkilogram". The SI symbol for the gram is the lowercase "g", standardised by ISO 80000-1 and the BIPM SI Brochure (9th edition, 2019). The older symbol "gm" — common in nineteenth-century scientific writing and still occasionally seen in US pharmacopoeial texts before the late twentieth century — is non-standard and not recognised by the BIPM, NIST, or any modern metrology authority. Because the gram is a submultiple of the kilogram, its modern definition rides on the kilogram's anchoring to the Planck constant: a gram is 1/1000 of the mass derived from h = 6.62607015 × 10⁻³⁴ J·s. The quantity 1 gram-force, equal to the gravitational force on one gram of mass at standard gravity (≈9.80665 mN), is a deprecated coherent unit not part of the SI; force in modern usage is expressed in newtons.

The gramme — and the unit's identity is the gramme rather than the kilogramme in the original metric texts — was first defined in the Loi du 18 germinal an III of 7 April 1795, the law that set out the Système Métrique Décimal in revolutionary France. The law fixed it as the absolute weight of a volume of pure water equal to the cube of the hundredth part of the metre, at the temperature of melting ice — the mass of one cubic centimetre of water at the ice point. The phrasing matters: an earlier 1793 provisional definition had set the grave at the mass of one cubic decimetre (one litre) of water at the freezing point, but the 1795 law moved to the much smaller cubic-centimetre reference and renamed the unit. For four years the gramme was the base unit of mass in the new French metric system, and the kilogramme its multiple. The relationship inverted with the deposition of the Kilogramme des Archives — the platinum standard built to realise the larger unit — at the National Archives in Paris on 22 June 1799 (4 messidor an VII), at which point the kilogramme assumed the role of base unit and the gramme was treated as its submultiple, where it has remained. Through the late nineteenth and twentieth centuries the gramme had a second life as the base unit of mass in the centimetre-gramme-second (CGS) system, formally adopted by the British Association for the Advancement of Science in 1874 and the workhorse of physics and astronomy until the 11th CGPM displaced it with the metre-kilogramme-second-based Système International in 1960. Since the SI redefinition of 2019 the gram is anchored to the Planck constant indirectly, via the kilogram — the same downstream chain that has held since the demotion of 1799.

Pharmaceuticals: the gram is the working unit for active pharmaceutical ingredient (API) bulk masses and for finished oral solid dosage forms in the United States Pharmacopeia (USP), the European Pharmacopoeia (Ph. Eur.) and the Japanese Pharmacopoeia, with monographs specifying allowable mass tolerances on tablet and capsule weight in grams or milligrams. Compounding pharmacies, hospital formularies and contract manufacturers track API inventory and dispensing in grams. Nutrition labelling: EU Regulation 1169/2011 (the Food Information to Consumers Regulation, in force since 13 December 2014) mandates per-100 g and per-portion nutrient declarations on every prepacked food sold in the EU, with macronutrients reported in grams and micronutrients in milligrams or micrograms. The US FDA Nutrition Facts panel, redesigned under the 2016 Nutrition Labeling rule and mandatory across the food industry by the 2020-2021 compliance window, denominates fat, carbohydrate, fibre and protein in grams per serving. Jewellery and precious metals: the gram is the global trade unit for gold at retail, with spot prices on the LBMA quoted in dollars per troy ounce but consumer bullion priced and bought by the gram in the great majority of jurisdictions outside the United States. The carat used for gemstone weight is defined as exactly 0.2 g by international agreement at the 4th CGPM in 1907. Specialty coffee: barista practice doses espresso shots between 16 and 22 g of dry ground coffee per double basket, with measured-mass extraction having displaced volume-based dosing in third-wave coffee since the early 2010s. Postal rates worldwide use gram thresholds for letter classes — 20 g, 50 g, 100 g — with national operators (Royal Mail, USPS, La Poste, Deutsche Post) all denominating their first-class letter rate breaks in grams.