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Friday, March 31, 2017

What's the conversion from avoirdupois to troy weight?

One of our patrons has a collection of silver, which, like all precious metals, is traditionally measured in troy weight. The standard weight used for almost everything else is called avoirdupois weight, from the Old French “avoir de peis,” which means “goods of weight,” and the two measurements are not equivalent. A troy ounce is a little larger than an avoirdupois ounce, but, because there are 16 ounces in an avoirdupois pound and only 12 in a troy pound, the troy pound is smaller.

In both units of measurement, the grain is the same: a little less than 65 milligrams (64.79891 to be exact). There are 437.5 grains in an avoirdupois ounce and 480 grains in a troy ounce. 1 troy ounce equals about 1.097 avoirdupois ounces (so, going the other way, 1 avoirdupois ounce equals about 0.911 troy ounces). 1 troy pound is about the same as 0.823 avoirdupois pounds. Reversed, that means that 1 avoirdupois pound equals 1.215 troy pounds.

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