One troy ounce of pure precious metal (gold or silver) contains 31.1034768 grams.

There can be a bit of confusion about this topic, particularly among people who don’t have a lot of experience buying or selling precious metals. Gold and silver are weighed in something called troy ounces, which is different from the standard ounce used to weigh most things.

When most people think of an ounce, they are using a measure of weight that is part of the avoirdupois system, which is based on a pound which contains 16 ounces.

In contrast, precious metals are weighed using a system of weights called the troy system, which is based on a pound which contains only 12 ounces.

There are 480 grains per troy ounce, 31.1 grams per troy ounce, or 20 pennyweights per troy ounce.

There are 437.5 grains per avoirdupois ounce, 28.35 grams per avoirdupois ounce, or 18.23 pennyweights per avoirdupois ounce.

Common Confusion

The difficulty for some buyers and sellers of gold and silver is that they are not familiar with the troy units of measurement, and will weigh a piece of gold using the avoirdupois system of weights and they will inaccurately claim that a piece is larger than it is.

Using the example of a gold nugget, let’s say that a gold prospector finds a nugget that weighs exactly 7.78 grams. Of course this we could use a coin, bar, or any other form of the metal for this example. (Natural gold nuggets are not refined, so they are not 99.999% pure gold like bullion, but regardless, they are still weighed in troy ounces).

The 7.78 gram nugget is ¼ troy ounces of gold using the proper conversion:

7.78 / 31.1 = 0.25 Troy Oz.

However, if a person mistakenly uses the avoirdupois system to weights, which contains only 28.35 grams in an ounce, the calculation would look like this:

7.78 / 28.35 = 0.274 Troy Oz.

This calculation is incorrect, and if the prospector stated that the nugget weighed 0.274 ounces it would be inaccurate. Yes, it weighs 0.274 avoirdupois ounces, but since precious metals are always calculated in troy ounces the actual weight should be stated as 0.25 troy ounces.

Obviously, this makes a difference in when someone makes the claim as to how much gold they have. While the gold nugget in our example is exactly the same ¼ troy ounce nugget in both calculations, using the avoirdupois metric to calculate the number in ounces will cause an error.

The moral of the story… if weighing precious metals in ounces, always be sure that it is done in TROY OUNCES, not avoirdupois ounces. It is a simple error to make, but is easily fixed by knowing and understanding the proper conversions when weighing precious metals. You don’t want to “short change” a buyer because you didn’t understand how precious metals like gold and silver are weighted. Not only would that be unethical, but it will also be embarrassing when someone points out the error to you.

Additional Reading about Gold Investing:

Investing in Gold Nuggets

Buy Natural Gold Nuggets

Gold Buying and Collecting

Selling Scrap Gold

History of the Troy Weight System for Precious Metals

Prior to the use of the metric system, the troy weight system was used all throughout Europe dating back to the 1400s. The true origin of the weight system is unknown, but many believe that it originated in Troyes in northeastern France.

England officially adopted the troy system for precious metals in 1527, and the United States followed suit by adopting it as the official precious metals standard in the Coinage Act of 1828. Today, single troy ounce bullion bars and coins are still the most popular form of precious metal for collectors.

Troy ounces are still recognized globally as the standard weight metric for all precious metals. Large mining companies pour large gold and silver bars often weighing up to 1000 troy ounces as a unit for transport and future sale on the market. Kilo bars are another common size used for gold, which weigh 1000 grams or 1 kilogram (32.15 troy ounces).