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How can a Troy Ounce be more than an ounce ?
Precious metals such as gold and silver are often sold by the troy ounce. Many people don’t realize that an ounce of gold (and other precious metals) is weighed in a different weighing system called "troy weights". A troy ounce is more than the typical ounce found at the grocery store.
In many English speaking countries there are two systems for measuring weight.
One, for precious metals, called the troy ounce.
The other is for Commodities such as sugar, grains, and the typical grocery items. It is called the avoirdupois ounce.
Here in the USA we often use the avoirdupois ounce for everyday weighing. Every time you step on the scale at home, or weight food found in the grocery, you use the avoirdupois ounce weight system. There are 16 avoirdupois ounces in a pound.
1 avoirdupois ounce = 437.5 grains, or 28.35 grams. 1 avoirdupois pound contains 16 (avoirdupois) ounces. This makes an Avoirdupois pound equal to about 453.6 grams, or the equivalent of 14.583 "troy ounces".
So what do you get when you buy an ounce ?
The troy ounce is heavier than the typical grocery store (avoirdupois) ounce used in the USA. When you purchase a 1 ounce silver or gold coin, you are receiving a "troy" ounce. If you put that same 1 troy oz. coin on a grocery store scale you will find that it weighs about 10% more than the food (avoirdupois) ounce you are familiar with. It will weigh about 1.1 avoirdupois ounces. A troy ounce is equivalent to about 31.1 grams.
Separately, the troy weight system has it's own definition of a "troy" pound. A "troy pound" contains 12 troy ounces or 373.24 grams (which is over 13 avoirdupois ounces).
It is thought that the troy ounce was named after a weight system used in Troyes France during the Middle Ages.
One troy ounce = 480 grains, or 31.10 grams.
There are also 20 pennyweights to a troy ounce.
A troy pound contains 12 troy ounces (over 13 avoirdupois ounces) and is equivalent to 373.24 grams. 32.15 troy ounces = 1 kilogram.
Consequently, a grocery store pound which weighs 16 avoirdupois ounces (or 453.59 grams) will contain about 14.58 troy ounces.
Perhaps an easy way to compare Troy ounces versus avoirdupois ounces is to look at how they weigh using the metric system
One troy ounce weighs
One avoirdupois ounce weighs
How to convert regular (avoirdupois) ounces to a troy ounce
Use this formula for an approximation to convert grocery store type weights to troy ounces.
Multiply # regular ounces X .912 to get approximate # troy ounces
Examples: A grapefruit weighs about 127.5 grams, or 4.5 avoirdupois ounces.
4.50 x .912 = 4.10 troy ounces.
Ten troy ounces are equivalent to about 10.96 regular ounces.
Because each troy ounce is heavier, it would take almost 11 regular ounces to equal ten troy ounces.
A troy ounce is heavier than an avoirdupois ounce
(1 avoirdupois ounce = 28.35 grams, a troy ounce = 31.1 grams)
A troy pound is lighter than a avoirdupois pound because it contains fewer ounces.
(Troy pound contains 12 troy ounces, avoirdupois pound contains 16 avoirdupois ounces. Even though the troy ounces are heavier, that difference is not enough make troy pounds heavier.)
Most gold and silver bullion type coins, including the US SILVER EAGLE dollar coins and the AMERICAN GOLD EAGLES coins, are weighed in troy oz.
28.35 x 16 = 453.6 g to a pound adv.
31.1 x 12 = 373.2 g to a pound troy.