Troy ounce vs ounce: ozt vs oz.

The two ounces

Both are defined exactly from the same grain, 64.79891 milligrams, which follows from the pound agreed by the U.S., the U.K., and four other countries in 1959. Here's the twist: the troy ounce is heavier than the ordinary ounce, but the troy pound is lighter than the ordinary pound, because it has only 12 ounces.

What "oz" means on a coin or bar

On bullion, "oz" means troy ounces. A "1 oz" American Silver Eagle holds one troy ounce of silver, and the law requires it to carry the inscription "1 Oz. Fine Silver." Standard abbreviations vary: the U.S. National Institute of Standards and Technology writes "oz t", and the trade also uses "ozt" and "t oz". The pennyweight (dwt), used by jewelers, is 1/20 of a troy ounce, 1.555 grams.

On a kitchen scale set to ounces, 1 troy ounce of gold reads about 1.1 oz. Weigh in grams to avoid the mix-up.

Where the troy ounce comes from

• The name: probably from Troyes, a city in France's Champagne region that held some of medieval Europe's great trade fairs. The Oxford English Dictionary says "probably", and a 1910 history of British weights suggests it may instead come from tron, an old word for a market balance.
• Medieval England: troy weight was used for gold and silver, and the "troy pound" appears in English law by 1414, according to the same 1910 history.
• Henry VIII: in 1526 or 1527 the Royal Mint dropped the older Tower pound for the troy pound: "all maner of golde and silver shall be wayed by the pounde Troy."
• The United States: an Act of Congress of May 19, 1828 made a brass troy pound, copied from the British standard in 1827, "the standard troy pound of the mint of the United States." U.S. coins were struck to it.
• Britain, 1878: the Weights and Measures Act left the troy pound off the list of legal units but kept the troy ounce, "Four hundred and eighty grains shall be an ounce troy", and let gold, silver, platinum, and precious stones be sold by it. Today the troy ounce is still legal for trade in the U.K., but only for precious metals.

Today

Gold and silver are priced in U.S. dollars per troy ounce worldwide: the London market, its benchmark prices, and COMEX futures in New York all use it. A COMEX gold contract is 100 troy ounces. China's Shanghai Gold Exchange quotes in yuan per gram, and a kilo is 32.15 troy ounces.

Sources

• NIST, Handbook 44, Appendix C (2026), and Special Publication 447, which reprints the 1959 notice.
• Act of May 19, 1828, 4 Stat. 277; 31 U.S.C. §5112.
• U.K. Weights and Measures Act 1878 and Weights and Measures Act 1985.
• Oxford English Dictionary, "troy"; C. M. Watson, British Weights and Measures (1910); LBMA, the troy ounce.

Weights and measures: Troy ounces, grams, and pennyweight · A gram of gold · Why silver dollars weigh 26.73 g · Why gold is measured in karats · All guides

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