What it means
The troy system is a set of weights used for precious metals, and the US National Institute of Standards and Technology (NIST) publishes the units in Handbook 44, Appendix C, listing 24 grains as one pennyweight, 20 pennyweights as one troy ounce, and 12 troy ounces as one troy pound. That means a troy ounce is 480 grains, and NIST's table lists the avoirdupois ounce, the ordinary ounce for most goods, as 437.5 grains.
The troy ounce is therefore larger: 480 / 437.5 = 1.0971, or about 9.7 percent heavier. NIST's precious metals conversion chart gives the metric equivalent: one troy ounce is 31.103 476 8 grams, one gram is about 0.032 150 75 troy ounces, and one pennyweight is about 1.555 173 84 grams.
The unit matters because precious metals are quoted per troy ounce, so if you see a gold price, the weight behind it is a troy ounce, not a regular ounce. Using the wrong ounce can change a calculation by nearly 10 percent.
NIST's chart also shows how to work from a quoted price: to get the price per pennyweight, divide the price per troy ounce by 20, and for a jewelry item weighed in pennyweights, multiply that price by the weight. The same chart explains converting a price per gram into a price per troy ounce.
NIST notes that state weights and measures programs have seen more scrap gold buyers and sellers using scales that read in grams and milligrams, which is one reason it published the conversion chart. If a buyer weighs in grams, convert the quote to the same unit before comparing offers.
This chart does not tell you the purity, since a weight in troy ounces refers to the whole item. NIST's own worked example, from a 2015 chart, uses a gold price of $1,181.93 per troy ounce, and dividing by 20 gives $59.10 per pennyweight.
Multiplying by a 6.85 pennyweight piece of jewelry gives $404.84. The price is dated and only shows the method, so use a current quote when you calculate.
A weight in grams for the same item is a separate measure. The metal content depends on its fineness, so check the stated purity too.
In practice
Real-world examples.
Example
A fictional gold coin is listed as one troy ounce. It weighs about 31.1035 grams. A regular ounce is about 28.35 grams, so the coin is heavier than an ounce bought at a market.
Example
A fictional buyer is quoted 2,000 per troy ounce for gold. Dividing by 20 gives 100 per pennyweight. A piece weighing 6.85 pennyweights is worth 685 on that illustrative quote, before any deduction for purity.
Example
A fictional scrap seller weighs a chain at 10 grams. Dividing by 31.1035 gives about 0.32 troy ounces. At 2,000 per troy ounce, that is about 643 of pure gold before purity.
Formula
Calculation
Grams = troy ounces x 31.1034768.
Price per gram = price per troy ounce / 31.1034768.
Example: $2,000 / 31.1034768 = about $64.30 per gram. Check: 480 grains / 437.5 grains = 1.0971, so a troy ounce is about 9.7 percent heavier than an avoirdupois ounce.
Second example. A fictional gold bar weighs 5 troy ounces. In grams, that is 5 x 31.1034768 = 155.517384 grams, and at $2,000 per troy ounce its value is 5 x $2,000 = $10,000, before any deduction for purity.
Third example, in pennyweights. A fictional ring weighs 3 pennyweights, which is 3 x 1.55517384 = about 4.67 grams, or 3 / 20 = 0.15 troy ounces. At $2,000 per troy ounce, the price per pennyweight is $2,000 / 20 = $100, so the gold in the ring is worth 3 x $100 = $300, again before purity.Case study
Seen in the real world.
This case study is fictional and illustrative. Ravi sees a gold bar advertised at 2,000 per ounce and a coin priced at the same figure. He assumes both quotes use the same ounce. He checks and finds that the quote uses troy ounces. NIST lists a troy ounce as 480 grains, against 437.5 grains for an avoirdupois ounce, so each troy ounce is about 1.0971 times the heavier common ounce.
He weighs a coin at 31.1 grams and converts it. 31.1 / 31.1034768 is about 1.0 troy ounce, so a 2,000 quote matches the coin. He divides 2,000 by 31.1034768 to get about 64.30 per gram and compares two offers on the same basis. The lesson is to confirm the unit and then convert all quotes to a common weight. The gap between ounces is large enough to cost real money.
Watch out
Common mistakes.
- Treating a troy ounce as a regular ounce, when it is about 9.7 percent heavier.
- Comparing a per-gram quote with a per-troy-ounce quote without converting, when the units differ.
- Assuming weight alone sets value, when the metal content depends on purity.
Questions
People also ask.
What is a troy ounce?
It is a unit of weight for precious metals, equal to 480 grains or about 31.1035 grams.
How is it different from a regular ounce?
A regular avoirdupois ounce is 437.5 grains. A troy ounce is 480 grains, so it is about 9.7 percent heavier.
How do I convert a per-troy-ounce price to per gram?
Divide by 31.1034768. For example, 2,000 per troy ounce is about 64.30 per gram.
From the founder's library
Take it further with the book.
Build your financial confidence beyond this definition. Shihan's full-length guide, Accounting Fundamentals, takes the same plain-English approach and turns it into a complete, practical playbook for non-finance managers, business owners and students - with chapter-end quiz answers and presentation slides included.
25% off with code MMHQ25, applied at checkout. Priced in USD - checkout may show the equivalent in your local currency.
View the book and save 25%Related