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Troy weight is a system of units of mass customarily used for precious metals, black powder, and gemstones.

Named after Troyes, France, the troy system of weights was known to exist in medieval times. One cubic inch of distilled water, at 62 °F (17 °C), and at a barometric pressure of 30 inches of mercury, was determined to weigh 252.458 troy grains (gr).[1]

Troy ounceEdit

The troy ounce (ozt) is 480 grains, somewhat heavier than an avoirdupois ounce (437.5 grains).[2] A grain is exactly 64.79891 mg; hence one troy ounce is exactly 31.1034768 g, about 10 percent more than the avoirdupois ounce, which is exactly 28.349523125 g. The troy ounce is the only ounce used in the pricing of precious metals, gold, platinum, and silver. The grain, which is identical in both the troy and avoirdupois systems, is used to measure arrow and arrowhead weights in archery and bullets and powder weights in ballistics. Grains were long used in medicine but have been largely replaced by milligrams.

Troy poundEdit

The troy pound (troy) is 5,760 gr (≈ 373.24 g, 12 ozt), while an avoirdupois pound is heavier at 7,000 gr (≈ 453.59 g).

There are 12 troy ounces per troy pound,[2] rather than 16 avoirdupois ounces (oz) in the avoirdupois pound (lb) as in the more common avoirdupois system. The avoirdupois pound is 14712 (≈ 14.583) troy ounces.

Scottish systemEdit

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 silver and gold.

ConversionsEdit

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
English pounds[note 1]
Unit Pounds Ounces Grains Metric
avdp. troy tower merc. lond. metric avdp. troy tower g kg
Avoirdupois 1 175144 3527 2827 3536 1011 16 14+712 15+59 7000 454 920
Troy 144175 1 1615 6475 45 34 13+29175 12 12+45 5760 373 38
Tower 2735 1516 1 45 34 710 12+1235 11+14 12 5400 350 720
Merchant 2728 7564 54 1 1516 78 15+37 14+116 15 6750 437 716
London 3635 54 43 1615 1 1415 16+1635 15 16 7200 467 715
Metric 1110 43 107 87 1514 1 17+35 16 17+17 7716 500 12
  1. English-metric ratios (in grey) are approximate.


The troy system was used in the Apothecaries' system, but with different further subdivisions.

Relationship to British coinageEdit

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.

See alsoEdit


ReferencesEdit

  1. Wightman, S.; Blanchard, William (1840). Wightman's Arithmetical Tables. Westminster: S. Wightman. OCLC 43196919. 
  2. 2.0 2.1 "Troy Ounce". WordNet 3.0, Dictionary.com. Princeton University. Retrieved on 2008-01-10.

External linksEdit

http://www.troy-ounce.com - essay on the history of the Troy Ounce with many links to further reading.

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