Troy weight

Troy weight

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).

Troy ounce

The troy ounce (ozt) is 480 grains, somewhat heavier than an avoirdupois ounce (437.5 grains). 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 pound

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

There are 12 troy ounces per troy pound, 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, since troy ounces are larger than avoirdupois ounces.

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.


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

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

Relationship to British coinage

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 also - essay on the history of the Troy Ounce with many links to further reading.


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