Know the definitions of pennyweight and karat. One pennyweight is simply 1/20 of an ounce. The karat is a way of telling the proportion of the metal in an item that is gold. Most jewelry isn't pure gold. Pure gold is very soft, so another metal is usually added to make the gold harder and durable. The number of karats tells you the proportion of gold, ranging from zero karats (no gold) to 24 karats (pure gold). Most jewelry ranges from 10 karats to 18 karats.
Find the karat number on a piece of gold jewelry. All gold jewelry carries a jeweler's mark stating the karat number. On very small pieces of jewelry, this can be tiny and you may need a magnifying glass to make it out.
To get an accurate reading, you need a scale designed for small weights. The easiest way to find a suitable scale is to go to a craft or hobby shop. A balance scale like students use in chemistry classes is ideal.
Figure the pennyweight of gold the jewelry contains. If you use a calculator, first divide the karat rating by 24 to convert to a decimal (for example, if an item is 18 karats, divide 18 by 24). Multiply this number by the weight in ounces. This is the total amount of actual gold. Finally, multiply this total amount in ounces by 20. The answer is the pennyweight of gold. An example is included as step 6. You can find more information about gold measures below.
Calculate the value of the gold in the jewelry. First check the price of gold. Divide the price in ounces by 20 to get the current value of a pennyweight of gold. Then multiply this figure by the pennyweight of the item to find its value.
Use this example as a guide to figure pennyweight of gold. If the jewelry is 18 karats and weighs 0.4 ounces, divide 18 by 24 (this equals 0.75) and multiply by the weight (0.4 ounces). The result (0.3 ounces) is the weight of gold in ounces. Multiply the weight by 20 to find the pennyweight (0.30 x 20 equals 6 pennyweight).