The numbers
| Unit | Grams | Gold at this weight |
|---|---|---|
| 1 grams | 1 | $141.02 |
| 1 pennyweight | 1.55517384 | $219.31 |
| 1 troy ounces | 31.1034768 | $4,386 |
| 1 ounces | 28.349523125 | $3,997 |
| 1 kilograms | 1000 | $141,021 |
| 1 grains | 0.06479891 | $9.14 |
A troy ounce is 31.1034768 grams. The ounce on a kitchen scale, the avoirdupois ounce, is 28.349523125 grams. The troy ounce is 9.714% heavier, and that percentage is the whole of the problem.
Why the mistake costs money
Suppose you weigh a bar on a kitchen scale, read 10 ounces, and price it at the gold quote of $4,386.25 an ounce. You have just valued 283.5 grams of gold at the price of 311 grams. The overstatement is $3,883 on that one calculation, and you walk into the shop expecting a number nobody is going to say.
The error runs the other way just as easily. A seller who converts a troy ounce price into a kitchen ounce weight underprices their own metal by the same 9.7%. The calculator on this site carries both units for exactly this reason: set the unit to match the scale you actually used, not to the word you heard quoted.
The pennyweight
A pennyweight, written dwt, is one twentieth of a troy ounce: 1.55517384 grams. It comes from the weight of a medieval English silver penny, and it survives because the American jewellery trade has always used it. Twenty pennyweights make a troy ounce, and 240 made a troy pound, which is where 240 pence to the pound sterling came from.
It matters today for one reason: a price quoted per pennyweight is 1.555 times the equivalent price per gram, so it sounds better than it is. A buyer offering a per pennyweight figure is not necessarily being deceptive, since that is genuinely how their trade quotes, but you cannot compare it to a gram quote without dividing. Do the division before you decide anything.
Troy pounds, grains and the rest
A troy pound is 12 troy ounces, 373.24 grams, and is lighter than the 453.59 gram avoirdupois pound despite the troy ounce being heavier. It is effectively obsolete. A grain, 0.06479891 grams, survives in the bullet and gemstone trades and occasionally on old scales. A carat, for stones, is 0.2 grams and has nothing to do with the karat of gold beyond an ancestral link to carob seeds.
The one rule
Weigh in grams, price in grams, and convert only at the end. Grams are unambiguous, every digital scale reads them, and the conversion to any other unit is a single multiplication that you can do once at the finish rather than repeatedly in the middle. Every figure on this site is computed in grams underneath, whatever unit you asked it to display.
Unit questions
What is a troy ounce?
A unit of 31.1034768 grams, used for precious metals and for essentially nothing else. Every gold, silver, platinum and palladium price you will see quoted is per troy ounce.
How many grams are in a troy ounce?
31.1034768 grams exactly. The ordinary avoirdupois ounce is 28.349523125 grams. The troy ounce is about 9.7% heavier.
Is a troy ounce heavier than a regular ounce?
Yes, by roughly a tenth. Confusingly, a troy pound is lighter than a regular pound, because a troy pound is 12 troy ounces rather than 16 ounces. That is why nobody in the metals trade uses pounds.
What is a pennyweight?
One twentieth of a troy ounce, 1.55517384 grams, abbreviated dwt. It is the traditional unit of the American jewellery trade and many buyers still quote in it.
How do I convert a price per pennyweight to a price per gram?
Divide by 1.55517384. A quote of $45 per pennyweight is $28.94 per gram. Always convert before comparing two offers, because a pennyweight price looks more than half as large again as the equivalent gram price.
Which unit should I weigh my gold in?
Grams, on a scale reading to 0.1 g. It is the finest common unit, most scales default to it, and every conversion is easy from there. Just make sure the calculator is set to grams as well.
Do jewellers use troy ounces?
They buy and sell metal in troy ounces and pennyweights and often weigh customer items in grams. Expect to hear all three in one conversation, and ask which unit any quoted price refers to.