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troy ounce

In the English-speaking world, at least as early as the 12ᵗʰ century – present, a unit of mass now used only for precious metals, = 31.103 476 8 grams = ¹⁄₁₂ troy pound = 480 troy grains, each traditionally the mass of a grain of barley. The troy ounce is identical to the apothecaries' ounce, formerly used for drugs. It is the last unit in troy weight remaining in regular use, and then only in the United States.

In the United Kingdom, when the troy pound was abolished in 1878, the troy ounce was retained for weighing precious stones and metals. The Weights and Measures Act of 1963 restricted use of the troy ounce to the weighing of precious metals. The Weights and Measures Act of 1985 redefined the troy ounce as ¹²⁄₁₇₅ of 0.45359237 kilogram.

In the sale of gold bullion, the terms fine ounce and standard ounce are sometimes used. A fine ounce is a troy ounce of almost pure gold; market statistics are reported in fine ounces. A standard ounce is 91.667% gold, approximately 22-carat. The British Royal Mint purchased in standard ounces, adjusting the payment according to the actual fineness of the metal.¹

1. Queensland Mining Guide. 1949 edition.
Brisbane: A.H. Tucker, Government Printer, 1949.

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