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Avoirdupois Ounces and Troy Ounces

The scrap silver value calculator and scrap gold value calculator allow users to enter the weight of their scrap precious metals in both troy and avoirdupois ounces. They then calculate the mass of the bullion in troy ounces. It's fairly common for newcomers to the subject of precious metals to be confused about the distinction between normal ounces (or avoirdupois ounces) and troy ounces. Troy ounces are the unit of mass in which precious metals are measured. An avoirdupois or normal ounce is what you would measure the flour to bake your cake in so if you used a set of kitchen scales to weigh your gold or silver then you would have an amount measured in avoirdupois ounces. One avoirdupois ounce is one sixteenth of an avoirdupois pound, with the pound defined as 7000 grains (a grain is 64.79891 milligrams). Therefore what we consider to be a normal ounce is 437.5 grains. In contrast, the troy ounce is one twelth of a troy pound which is 5760 grains, making each troy ounce equal to 480 grains. In metric terms a normal or avoidupois ounce is 28.3495231 grams while a troy ounce is 31.1034768 grams, or 9.7% heavier.

The troy weight system is probably named after the city of Troyes in France with the system being developed fully sometime in the middle ages. It's likely that the origins of the troy system date back even further, with the Romans also having a pound that measured 12 ounces (but the ounces were different to both the troy and avoirdupois ounce). The name 'ounce' derives directly from the Latin word 'uncia'. 'Uncia' translates literally as 'a twelth part', and it's no surprise that there were 12 uncias in the Roman pound (or libra in Latin). An interesting bit of trivia is that the £ symbol from the old pounds, shillings, and pence currency system is derived from the first letter of the word 'libra'.

It's well worth taking a look at the units of measure used by the troy weight system. Under the troy system there are 24 grains to the pennyweight (dwt), 20 pennyweight to the troy ounce, and 12 troy ounces to the troy pound. Thus there are 240 penny weight to the pound. This happens to be the same system used by the old English pounds/shillings/pence system. The English penny was originally exactly one pennyweight of silver, a shilling was 12 pennyweight, and 20 shillings (or 240 pennyweight) of silver was an English pound!

An interesting (and fairly useless) bit of trivia is that the grain, as the basis of troy weight system is defined by the mass of one cubic inch of distilled water at 17 degrees centigrade and at an atmospheric pressure of 30 inches of mercury. And how many grains does this weigh? 252.458! It's also worth noting that the grain, is exactly the same mass under the troy and avoirdupois system and that grains are still used today to measure the weight of propellant in firearms cartridges and to weigh arrowheads.


The Free Dictionary
The Troy Ounce - Wikipedia
Troy Weight - Wikipedia
What is a Troy Ounce

Page last modified: April 10, 2011