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Troy weight is a system ofunits ofmass that originated in theKingdom of England in the15th century[1] and is primarily used in theprecious metals industry. The troy weight units are thegrain, thepennyweight (24 grains), the troy ounce (20 pennyweights), and the troy pound (12 troy ounces). The troy grain is equal to thegrain unit of theavoirdupois system, but the troy ounce is heavier than theavoirdupois ounce, and the troy pound is lighter than theavoirdupois pound. Legally, one troy ounce (oz t) equals exactly 31.1034768grams.
Troy weight is generally supposed to take its name from the French market town ofTroyes where English merchants traded at least as early as the early9th century.[2][3] The nametroy is first attested in 1390, describing the weight of a platter, in an account of the travels in Europe of theEarl of Derby.[2][4]
Charles Moore Watson (1844–1916) proposes an alternative etymology:The Assize of Weights and Measures (also known asTractatus de Ponderibus et Mensuris), one of thestatutes of uncertain date from the reign of eitherHenry III orEdward I, thus before 1307, specifies "troni ponderacionem"—which the Public Record Commissioners translate as "troy weight". The wordtron refers to markets.[citation needed] Wright'sThe English Dialect Dictionary lists the wordtroi as meaning abalance, related to the alternate form 'tron' which also means market or the place of weighing. From this, Watson suggests that 'troy' derives from the manner of weighing by balance precious goods such as bullion or drugs; in contrast to the word 'avoirdupois' used to describe bulk goods such as corn or coal, sometimes weighed in ancient times by a kind ofsteelyard called theauncel.[5]
Troy weight referred to theTower system; the earliest reference to the modern troy weights is in 1414.[5][6]
The origin of the troy weight system is unknown. Although the name probably comes from theChampagne fairs atTroyes, in northeastern France.[7] English troy weights were nearly identical to the troy weight system ofBremen. (The Bremen troy ounce had a mass of 480.8 British Imperial grains.)[8]
Many aspects of the troy weight system were indirectly derived from theRoman monetary system. Before they used coins,early Romans used bronze bars of varying weights as currency. Anaes grave ("heavy bronze") weighed onepound. One twelfth of anaes grave was called anuncia, 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.[9] 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.[1] The British Imperial system of weights and measures (also known asImperial units) was established in 1824, prior to which the troy weight system was a subset of pre-ImperialEnglish units.
The troy ounce in modern use is essentially the same as the British Imperial troy ounce (1824–1971), adopted as an official weight standard for United States coinage by act ofCongress on May 19, 1828.[10] 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 theAct of Union which created theKingdom of Great Britain.) Troy ounces have been used in England since the early 15th century, 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.[8]
The troy ounce and grain were also part of theapothecaries' system. This was long used in medicine, but has been largely replaced by themetric system (milligrams).[11] The only troy weight in widespread use is the British Imperial troy ounce and its American counterpart. Both are based on a grain of 0.06479891 gram (exact, by definition), with 480 grains to a troy ounce (compared with437+1⁄2 grains for an ounce avoirdupois). The British Empire abolished the 12-ounce troy pound in the 19th century. It has been retained, though rarely used, in the American system. Larger amounts of precious metals are conventionally counted in hundreds or thousands of troy ounces, or in kilograms.
Troy ounces have been and are still often used in precious metal markets in countries that otherwise useInternational System of Units (SI).[12][13] However, thePeople's Bank of China – which had been using troy measurements in mintingGold Pandas since 1982 – from 2016 specifies Chinese bullion coins in an integer numbers of grams.[14]
The troy pound (lb t) consists of twelve troy ounces[15] and thus is5760 grains (373.24172 grams). (An avoirdupois pound is approximately 21.53% heavier at7000 grains (453.59237 grams), and consists of sixteen avoirdupois ounces).
A troy ounce weighs 480grains.[15] Since the implementation of theinternational yard and pound agreement of 1 July 1959, the grain measure is defined as precisely64.79891 milligrams.[16]: C-19 [17] Thus one troy ounce = 480 grains ×0.06479891 grams/grain =31.10347680 grams. Since the ounce avoirdupois is defined as 437.5 grains, a troy ounce is exactly480⁄437.5 =192⁄175 or about 1.09714 ounces avoirdupois or about 9.7% more. The troy ounce for trading precious metals is considered to be sufficiently approximated by 31.10 g in EU directive 80/181/EEC.[18]
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 grams. The divisions are identical to the tower system.[9]
Thepennyweight symbol isdwt. One pennyweight weighs 24 grains, and 20 pennyweights make one troy ounce.[15] 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 theoriginal pound-sterling. However, prior to 1526, the Englishpound sterling was based on thetower pound, which is15⁄16 of a troy pound. Thed indwt stands fordenarius, the ancient Roman coin that equates loosely to a penny. The symbold 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, andd, respectively.
There is no specific 'troy grain'. AllImperial systems use the same measure of mass called a grain (historically ofbarley), each weighing1⁄7000 of an avoirdupois pound (and thus a little under 65 milligrams).[a]
Mint masses, also known asmoneyers' masses, were legalized by Act of Parliament dated 17 July 1649 entitledAn 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.[19][20]
| Unit | Grains | Grams (exact) |
|---|---|---|
| Troy pound (12 troy ounces) | 5,760 | 373.24172 160 |
| Troy ounce (20 pennyweights) | 480 | 31.10347 680 |
| Pennyweight | 24 | 1.55517 384 |
| Grain | 1 | 0.06479 891 |
| Unit | Pounds | Ounces | Grains | Metric | |||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Avdp. | Troy | Tower | Merchant | London | Metric | Avdp. | Troy | Tower | Troy | Tower | g | kg | |||||||||
| 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 theapothecaries' system but with different further subdivisions.
The received opinion is that it took its name from a weight used at the fair of Troyes in France
...the great fairs established for all Europe the weight-standardTroyes, whence...Troy...
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.