When weighing gold and other precious metals, you must use a different method or weights, called troy ounces. These are measured in units of pennyweights or gold grams.
There are 20 pennyweights in a troy ounce, commonly abbreviated as DWT. The pennyweight was the actual weight of a medieval British silver penny and has remained the traditional method of weight for gold and other precious metals.
There are 31.1 gold grams in a troy ounce.
Food and Postage Grams
In common food and postage weights, there are 28 g per ounce.
A troy ounce weighs about 10 percent more than 1 oz. of food and is used for weighing precious metals. It is called a troy ounce after the system of weights used in Troye, France, in medieval times.
To convert gold grams to pennyweight, multiply by .64. To convert pennyweight to gold grams, multiply by 1.555.