Troy weight is a system of units of mass that originated in 15th-century England, and is primarily used in the precious metals industry. The troy weight units are the grain, the pennyweight (24 grains), the troy ounce (20 pennyweights), and the troy pound (12 troy ounces). The troy grain is equal to the grain unit of the avoirdupois system, but the troy ounce is heavier than the avoirdupois ounce, and the troy pound is lighter than the avoirdupois pound. One troy ounce (oz t) equals exactly 31.1034768 grams.
Troy weight probably takes its name from the French market town of Troyes where English merchants traded at least as early as the early 9th century. The name troy is first attested in 1390, describing the weight of a platter, in an account of the travels in Europe of the Earl of Derby.
Charles Moore Watson (1844–1916) proposes an alternative etymology: The Assize of Weights and Measures (also known as Tractatus de Ponderibus et Mensuris), one of the statutes of uncertain date from the reign of either Henry III or Edward I, thus before 1307, specifies "troni ponderacionem"—which the Public Record Commissioners translate as "troy weight". The word troni refers to markets. Watson finds the dialect word troi, meaning a balance in Wright's The English Dialect Dictionary. Troy weight referred to the tower system; the earliest reference to the modern troy weights is in 1414.
Many aspects of the troy weight system were indirectly derived from the Roman monetary system. The Romans used bronze bars of varying weights as currency. An aes grave ("heavy bronze") weighed one pound. One twelfth of an aes grave was called an uncia, or in English, an "ounce". Before the adoption of the metric system, many systems of troy weights were in use in various parts of Europe, among them Holland troy, Paris troy, etc. Their values varied from one another by up to several percentage points. Troy weights were first used in England in the 15th century, and were made official for gold and silver in 1527. The British Imperial system of weights and measures (also known as Imperial units) was established in 1824, prior to which the troy weight system was a subset of pre-Imperial English units.
The troy ounce in use today is essentially the same as the British Imperial troy ounce (1824–1971), adopted as an official weight standard for United States coinage by Act of Congress on May 19, 1828. The British Imperial troy ounce (known more commonly simply as the imperial troy ounce) was based on, and virtually identical with, the pre-1824 British troy ounce and the pre-1707 English troy ounce. (1824 was the year the British Imperial system of weights and measures was adopted, 1707 was the year of the Act of Union which created the Kingdom of Great Britain.) Troy ounces have been used in England since about 1400 and the English troy ounce was officially adopted for coinage in 1527. Before that time, various sorts of troy ounces were in use on the continent.
The only troy weight in widespread use today is the British Imperial troy ounce and its American counterpart. Both are currently based on a grain of 0.06479891 gram (exact, by definition), with 480 grains to a troy ounce (compared with 437+1⁄2 grains for an ounce avoirdupois).
The British Empire abolished the 12-ounce troy pound in the 19th century, though it has been retained (although rarely used) in the American system. Larger amounts of precious metals are conventionally counted in hundreds or thousands of troy ounces (or in kilograms).
The origin of the troy weight system is unknown. Although the name probably comes from the Champagne fairs at Troyes, in northeastern France, the units themselves may be of more northern origin. English troy weights were nearly identical to the troy weight system of Bremen. (The Bremen troy ounce had a mass of 480.8 British Imperial grains.)
An alternative suggestion is that the weights come from the Muslim domains by way of the Gold Dirhem (47.966 British Imperial grains), in the manner that King Offa's weights were derived from the silver Dirhem (about 45.0 British grains).
According to Watson, troy relates to a dialect word troi (balance). Then troy weight is a style of weighing, like auncel or bismar weights, or other kindred methods. The troy weight then refers to weighing of small precious or potent goods, such as bullion and medicines.
Use in other countries
Troy ounces are still often used in precious metal markets in countries that otherwise use International System of Units (SI), except in East Asia. The People's Bank of China, in particular, which has never historically used troy measurements, in 2016 began minting Gold Pandas in integer numbers of grams.
Units of measurement
Troy ounce (oz t) 
A troy ounce weighs 480 grains. Since the implementation of the international yard and pound agreement of 1 July 1959, the grain measure is defined as precisely 64.79891 milligrams.: C-19  Thus one troy ounce = 480 grains × 0.06479891 grams/grain = 31.10348 grams. Since the ounce avoirdupois is defined as 437.5 grains, a troy ounce is exactly 480/437.5 or about 1.09714 ounces avoirdupois or about 9.7% more.
Troy pound (lb t) 
The troy pound (lb t) consists of twelve troy ounces and thus is 5760 grains (373.24172 grams). (An avoirdupois pound is approximately 21.53% heavier at 7000 grains, and consists of sixteen avoirdupois ounces).
The pennyweight symbol is dwt. One pennyweight weighs 24 grains and 20 pennyweights make one troy ounce. Because there were 12 troy ounces in the old troy pound, there would have been 240 pennyweights to the pound (mass) – just as there were 240 pennies in the original pound-sterling. However, prior to 1526, English pound sterling was based on the tower pound, which is 15⁄16 of a troy pound. The d in dwt stands for denarius, the ancient Roman coin that equates loosely to a penny. The symbol d for penny can be recognized in the form of British pre-decimal pennies, in which pounds, shillings, and pence were indicated using the symbols £, s, and d, respectively.
Mint masses, also known as moneyers' masses were legalized by Act of Parliament dated 17 July 1649 entitled An Act touching the monies and coins of England. A grain is 20 mites, a mite is 24 droits, a droit is 20 perits, a perit is 24 blanks.
This section needs additional citations for verification. (July 2021)
In Scotland, the Incorporation of Goldsmiths of the City of Edinburgh used a system in multiples of sixteen. (See Assay-Master's Accounts, 1681–1702, on loan from the Incorporation to the National Archives of Scotland.)[better source needed] There were 16 drops to the troy ounce, 16 ounces to the Scottish troy pound, and 16 pounds to the stone. The Scots had several other ways of measuring precious metals and gems, but this was the common usage for gold and silver.
The Scottish pound was 7,716 grains, but after the union of Scotland with England, was reduced to 7,680 grains. This divides to 16 ounces, each of 16 drops, each of 30 grains. The reduction made the ounce and grain equal in mass to the English standard.
The Dutch troy system is based on a mark of 8 ounces, the ounce of 20 engels (pennyweights), the Engel of 32 as. The mark was rated as 3,798 troy grains or 246.084 grammes. The divisions are identical to the tower system.
|Troy pound (12 troy ounces)||5,760||373.24172 16|
|Troy ounce (20 pennyweights)||480||31.10347 68|
|Avoirdupois||1||175/144||= 1.21527||35/27||= 1.296||28/27||= 1.037||35/36||= 0.972||≈ 0.9072||16||14+7/12||= 14.583||15+5/9||= 15.5||7,000||9,955+5/9||≈ 454||≈ 5/11|
|Troy||144/175||≈ 0.8229||1||16/15||= 1.06||64/75||= 0.853||4/5||= 0.8||≈ 0.7465||13+29/175||≈ 13.17||12||12+4/5||= 12.8||5,760||8,192||≈ 373||≈ 3/8|
|Tower||27/35||≈ 0.7714||15/16||= 0.9375||1||4/5||= 0.8||3/4||= 0.75||≈ 0.6998||12+12/35||≈ 12.34||11+1/4||= 11.25||12||5,400||7,680||≈ 350||≈ 7/20|
|Merchant||27/28||≈ 0.9643||75/64||= 1.171875||5/4||= 1.25||1||15/16||= 0.9375||≈ 0.8748||15+3/7||≈ 15.43||14+1/16||= 14.0625||15||6,750||9,600||≈ 437||≈ 7/16|
|London||36/35||≈ 1.029||5/4||= 1.25||4/3||= 1.3||16/15||= 1.06||1||≈ 0.9331||16+16/35||≈ 16.46||15||16||7,200||10,240||≈ 467||≈ 7/15|
|Metric||≈ 1.1023||≈ 1.3396||≈ 1.4289||≈ 1.1431||≈ 1.0717||1||≈ 17.64||≈ 16.08||≈ 17.15||7,716||10,974||= 500||= 1/2|
The troy system was used in the apothecaries' system, but with different further subdivisions.
Relationship to British coinage
This section needs additional citations for verification. (July 2021)
King Offa's 8th century currency reform replaced the sceat with the silver penny. This coin was derived from half of a silver dirhem. The masses were then derived by a count of coins, by a mix of Charlemagne and Roman systems. An ounce was set as twenty pence, and a pound to twelve ounces or 240 silver pence. In practice, the weights of the coins was not consistent and 240 of them seldom added up to a full pound; there were no shilling or pound coins and the pound as used only as an accounting convenience.
Later kings debased the coin, both in weight and fineness. The original pound divided was the tower pound of 5,400 grains, but a later pound of 5,760 grains displaced it. Where once 240 pence made a tower pound (and 256 make a troy pound), by the time of the United Kingdom Weights and Measures Act of 1824, a troy pound gives 792 silver pence, still minted as such as Maundy Money.
- Bullion coin
- Carat (unit)
- Conversion of units
- English units
- Fluid ounce
- Grain (unit)
- Imperial units
- Mark (weight)
- Tola (unit)
- United States customary units
- Because an avoirdupois pound weighs 7000 grains and a pound weighs exactly 0.45359237 kg (see International Yard and Pound Agreement). Thus one grain weighs 453.59237/7 mg.
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The received opinion is that it took its name from a weight used at the fair of Troyes in France
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…the great fairs established for all Europe the weight-standard Troyes, whence…Troy…
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The French livre contained, in the time of Charlemagne, a pound, Troyes weight, of silver of a known finess. The fair of Troyes in Champaign was at that time frequented by all the nations of Europe, and the weights and measures of so famous a market were generally known and esteemed.
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- A new English dictionary on historical principles: founded mainly on the materials collected by the Philological Society. Philological Society (Great Britain). 1891. p. 675 – via Clarendon Press.
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- Lowther, Ed (14 February 2014). "A short history of the pound". BBC News. BBC.
Anglo-Saxon King Offa is credited with introducing the system of money to central and southern England in the latter half of the 8th Century, overseeing the minting of the earliest English silver pennies – emblazoned with his name. In practice they varied considerably in weight and 240 of them seldom added up to a pound. There were at that time no larger denomination coins – pounds and shillings were merely useful units of account.