How to Calculate Gold Weight
Determining how much gold is in a coin, bar or piece of jewelry is more complicated than it might seem at first glance. That's because gold is rarely pure, with a few exceptions such as American Buffalo gold coins. Pure gold is very soft and is easily damaged. To protect the gold, it is alloyed (combined) with another metal to make it hard and durable. You can easily calculate gold weight, once you know how to find the proportion of gold in an item and express it in ounces or pennyweight (a jeweler's measure of gold weight, abbreviated DWT).
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Things You'll Need
- Precise scale
- Magnifying glass (for small pieces of jewelry)
- Calculator
Instructions
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1
Find the proportion of gold a coin or bar contains. For gold bars, this is easy because it will be stamped into the metal. This may be stated as a percentage or in karats. One karat equals 1/24 part of gold. If the bar is 22-karat, divide 22 by 24 and multiply by 100 to get the percentage (22-karat equals 91.67%). For coins there is no imprint, so you have to look up the specifications in a catalog, which you can find at any coin shop or on coin collector websites.
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2
Look for a jeweler's mark stating the karat weight. This will be stamped or engraved somewhere on any piece of jewelry. It may be so small you'll need a magnifying glass to read it. Jewelry is always listed in karats, and you convert to a percentage as in Step 1.
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3
Weigh the gold. Because the amounts are usually small (especially for jewelry), you'll need a precision scale designed for small quantities. A chemistry lab scale will do nicely. To calculate gold weight in ounces, multiply the total weight by the percentage of gold in the item to find the amount of actual gold. For example, a 22-karat piece of jewelry weighing 1.20 ounces contains 1.20 ounces times 91.67%, or 1.10 ounces of gold.
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4
Convert ounces to pennyweight for jewelry. A pennyweight is equal to 1/20 ounce of gold. Jewelers use this measure because the quantity of gold in most jewelry is very small. Calculate gold weight in pennyweight by multiplying the weight in ounces by 20. For example, the piece of jewelry in Step 3 has 1.10 ounces of gold. Multiply this by 20 and you have 22 pennyweight of gold.
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Tips & Warnings
Gold jewelry often contains materials other than the gold alloy, such as jewels or other precious metals. If you can discover the weight of these materials and subtract them, you can accurately calculate gold weight. If not, or if the item is complicated, it will take a professional jeweler to find the exact weight of gold.
