Troy weight is a system of units of mass customarily used for precious metals, gemstones, and black powder.
There are 12 troy ounces per troy pound,[1] rather than the 16 ounces per pound found in the more common avoirdupois system. The troy ounce is 480 grains, compared with the avoirdupois ounce, which is 437-1/2 grains. Both systems use the same grain of exactly 0.06479891 gram.[2] Although troy ounces are still used to weigh gold, silver and gemstones, the troy pound is no longer used.
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The name "troy" is first attested in 1390. Troy weight probably takes its name from the French market town of Troyes in France where English merchants traded at least as early as the early 9th century.[3][4]
Prior to 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.[5] Their values varied from one another by up to several percentage points.
The only system of troy weights in widespread use today is the British Imperial, more commonly known as Imperial, and its American counterpart. Both the British Imperial and American troy weights 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). However, the British Empire abolished the 12-ounce troy pound in the 19th century, while it has been retained (although rarely used) in the American system.
In the USA, troy weights are part of the United States customary units.
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.
Troy weights were first used in England in the 1400s, and were made official for gold and silver in 1527.[6]
The origin of the troy weight system is unknown. Though the name probably comes from the Champagne fairs at Troyes, in northeastern France, the units themselves are probably of more northern origin. English troy weights were apparently derived from the nearly identical troy weight system of Bremen. (The Bremen troy ounce had a mass of 480.8 British Imperial grains.[3]
The troy pound (troy) is 5,760 grains (≈ 373.24 g, 12 oz t), while an avoirdupois pound is ≈21.53% heavier at 7,000 grains (≈ 453.59 g).
One troy ounce (oz t) is equal to 31.1034768 grams.
The pennyweight symbol is dwt. There are 24 grains in 1 dwt, and 20 dwt in one troy ounce. Because there were 12 troy ounces in the old troy pound, there would have been 240 pennyweights to the pound—the basis of the fact that the old British pound (currency) contained 240 pence. (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, referred to in the New Testament, that equates loosely to a penny. The symbol d for penny can be recognized in the old-style notation for British pennies, in which a quantity of money expressed in pounds, shillings, and pence was indicated using the symbols £, s, and d, respectively. For example, £6.11s.8d indicated six pounds, eleven shillings, and eight pence.
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.) Thus, there were 16 drops to the troy ounce, 16 ounces to the troy pound, and 16 pounds to the troy stone. The Scots had several other ways of measuring precious metals and gems, but this was the common usage for gold and silver.
| Unit | Grains | Grams |
|---|---|---|
| Troy pound (12 troy ounces) | 5,760 | 373.241 721 6 |
| Troy ounce (20 pennyweights) | 480 | 31.103 476 8 |
| Pennyweight | 24 | 1.555 173 84 |
| Grain | 1 | 0.064 798 91 |
| Unit | Pounds | Ounces | Grains | Metric | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| avdp. | troy | tower | merc. | lond. | metric | avdp. | troy | tower | troy | tower | g | kg | |
| Avoirdupois | 1 | 175⁄144 | 35⁄27 | 28⁄27 | 35⁄36 | 10⁄11 | 16 | 14 7⁄12 | 15 5⁄9 | 7000 | 9955 5⁄9 | 454 | 9⁄20 |
| Troy | 144⁄175 | 1 | 16⁄15 | 64⁄75 | 4⁄5 | 3⁄4 | 13 29⁄175 | 12 | 12 4⁄5 | 5760 | 8192 | 373 | 3⁄8 |
| Tower | 27⁄35 | 15⁄16 | 1 | 4⁄5 | 3⁄4 | 7⁄10 | 12 12⁄35 | 11 1⁄4 | 12 | 5400 | 7680 | 350 | 7⁄20 |
| Merchant | 27⁄28 | 75⁄64 | 5⁄4 | 1 | 15⁄16 | 7⁄8 | 15 3⁄7 | 14 1⁄16 | 15 | 6750 | 9600 | 437 | 7⁄16 |
| London | 36⁄35 | 5⁄4 | 4⁄3 | 16⁄15 | 1 | 14⁄15 | 16 16⁄35 | 15 | 16 | 7200 | 10240 | 467 | 7⁄15 |
| Metric | 11⁄10 | 4⁄3 | 10⁄7 | 8⁄7 | 15⁄14 | 1 | 17 3⁄5 | 16 | 17 1⁄7 | 7716 | 10974 | 500 | 1⁄2 |
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The troy system was used in the apothecaries' system, but with different further subdivisions.
The troy system was the basis for the pre-decimalisation British system of coinage introduced by Henry II of England, in which the penny was literally one pennyweight of silver. One pound sterling was equal to twenty shillings, with each shilling equal to twelve pennies. Thus, one pound sterling equals 240 pennyweights, or one troy pound of sterling silver.
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