What Is the Difference Between Standard Ounces & Troy Ounces?
A troy ounce weighs more than a standard (also called an avoirdupois) ounce, but a troy pound weighs less than a standard pound. Definitions are important when confusion can mean hundreds of dollars of difference per ounce when buying or selling precious metals.
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Troy Ounce
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A troy ounce is more than 9.7 percent heavier than a standard or avoirdupois ounce: There are 31.103 g per troy ounce, versus 28.349 g in a standard ounce. A troy pound weighs less than a standard pound, however, because there are only 12 ounces in a troy pound, compared to 16 ounces in a standard or international pound. A troy ounce is used when weighing precious metals such as gold, platinum and silver; gemstones; and some other products. For all other weight measurements, the standard ounce is used.
Troy History
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The Troy ounce is believed to have originated from the need to have a uniform system of weight measurements for large fairs held in Troyes, France. In his 1776 book "The Wealth of Nations," Adam Smith wrote that the French livre (pound) contained a pound (12 oz.), Troyes weight, "of silver of a known fineness" as early as the reign of Charlemagne (late 700s). These weights and measures were "generally known and esteemed." Traders had a fixed Troy weight of silver with a known fineness, making it easier to trade and pay for goods.
Avoirdupois History
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According to Sizes, "avoirdupois" originally referred to "goods of weight." These were items sold in bulk, with weighing done on a balance. The avoirdupois pound and ounce were used in the wool trade, the most heavily traded international commodity in the 14th century. The same market balance that determined wool weights was used to weigh other commodities, leading to the adoption of the "standard" pound and "standard" ounce to weigh things other than precious metals.
American Weights
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In 1827, then United States ambassador to the United Kingdom and former U.S. treasury secretary Albert Gallatin obtained a brass troy weight that was a copy of the British Imperial troy pound. He shipped this under seal to the mint in Philadelphia. In 1828, Congress adopted this weight as the official standard Troy pound of the mint, creating a standard weight for coinage. The avoirdupois pound was later set at 7000/5760th of the mint's troy pound, with 16 ounces to the avoirdupois pound and 12 ounces for the troy pound.
Warning
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The 9.7 percent greater weight of a troy ounce compared to the weight of a standard ounce makes for wide price differences when buying, selling and appraising precious metals. Find out if a scale is weighing in troy or standard ounces. Determine whether prices are quoted as troy or standard. At a hypothetical price of $1,000 per ounce, the difference between a troy ounce and standard ounce of metal is $97 (0.097 x $1,000 = $97).
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References
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