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Guide to Precious Metals and Gemstones Weights and Measures

This page explains weights and measures used for gold, silver, and precious stones. Discusses grams, pennyweight, troy ounces, carats and karats. Includes handy conversion charts and valuing tips. If you like collecting jewelry or silver, you should have a copy of this reference. Uploaded by the author, a jewelry dealer and gemologist.

Troy Weight - gold and silver are measured in Troy weight, a system that includes pennyweights, ounces and pounds. The ounces and pounds do not equal the Avordupois or customary U.S. system that other common goods are measured in. For example a troy ounce weights about 10 percent more than a Avordupois ounce.

24 grains = 1 pennyweight = 1.5552 grams
20 pennyweight = 1 troy ounce = 31.1035 grams
12 ounces = 1 pound troy = 373.24 grams.
Gold is also commonly measured in metric grams.

Precious stones are measured in carats with a carat being 200 milligrams or 1/5 of a gram. Carats are further broken down into "points" which are 1/00 of a carat. Note that Karat with a "K" is a measure of the purity of a gold alloy and is discussed separately below.

The conversion table below will show you how to convert from avordupois to troy to metric, and to and from units within each of the systems.

The table can be especially handy if you don't have the proper scale to use. However you must be sure your scale is sensitive enough for the job. It's fine to use an accurate postal scale to weigh a piece of silver that weighs several ounces and then make the conversion to troy weight, but don't try to weigh a piece of gold that weighs only a few grams using a postal scale.

Similarly, if you are trying to weigh a diamond to the nearest 1/100 of a carat, which is how diamonds are measured, use a scale with carat precision or a gram scale that is accurate to at least .002 gram. Note that when converting from one system to another, there will be slight rounding errors of a fraction of a percent.

Weight Conversion Chart for Silver, Gold and Gems

To convert:

Avordupois Ounces to Troy ounces multiply by .91
Avordupois Ounces to pennyweight multiply by 18.2
Avordupois ounces to grams multiply by 28.35
Troy ounces to grams multiply by 31.1
Troy ounces to pennyweight multiply by 20
Troy ounces to pounds troy, divide by 12
Pennyweight to grams multiply by 1.56
Pennyweight to troy ounces multiply by .05 (or divide by 20)
Grams to pennyweight multiply by .64
Grams to troy ounces multiply by .032
Grams to carats multiply by 5
Carats to grams multiply by .2 (or divide by 5)
Carats to "Points" multiply by 100.
"Points" to carats multiply by .01 (or divide by 100)

Karat designation for gold refer to the number of parts per 24 that is pure gold. Thus 14K is 14/24 pure gold, while 10K is 10/24 pure gold. Please note, however, that if a piece of jewelry is marked something like "1/20 14K G.F" this means that it is gold-filled, and 1/20 of the weight of the piece is the layer of 14K gold on the outside of the piece. In this case you must divide the weight of the piece by 20, and then apply the formula in the chart below.
The following chart shows how to find the weight of pure gold present in each common alloy of gold used in jewelry.