Troy Ounce vs Ounce

Gold and silver prices are quoted in a unit most people never use at home: the troy ounce. This page explains how it differs from the ounce on a kitchen scale, using the federal measurement tables. It is part of our guide to Gold and Precious Metals Basics for IRA Investors. It is general education, not advice, and it makes no price predictions.

What is a troy ounce?

A troy ounce is a unit of mass used for precious metals. The National Institute of Standards and Technology (NIST) lists it as "1 troy ounce (oz t)" in its conversion tables, equal to 480 grains and 31.103 476 8 grams (NIST Handbook 44, Appendix C). A grain is a very small old unit of mass. NIST states that 1 grain is 64.798 91 milligrams "(exactly)".

What is the ordinary ounce, and how is it different?

The other ounce, the avoirdupois ounce, (say "av-er-duh-POYZ"). NIST lists it as 437.5 grains and 28.349 523 125 grams. So the two ounces are not the same size:

• A troy ounce has 480 grains. An avoirdupois ounce has 437.5 grains.
• NIST's equivalents table says a troy ounce is "1.097 avoirdupois ounces." It gives the avoirdupois ounce as "0.911 troy or apothecaries ounce."

In plain terms, a troy ounce is a little under 10% heavier than the ounce you know.

How do the units convert?

Source: NIST Handbook 44, Appendix C. NIST also lists the troy pound as 12 troy ounces in its tables. The troy pound is rarely used in modern buying, so you are unlikely to meet it.

Rounded to four decimals, 1 troy ounce is 31.1035 grams. For exact work, such as a contract, use the full NIST value.

What does "one ounce" mean on a U.S. Mint coin?

On the U.S. Mint's bullion coin page, the unit is the troy ounce. For the American Eagle silver coin, the Mint says: "Each coin's weight and fineness of one troy ounce of .999 fine silver is guaranteed by the United States Government." Its specification line gives the weight as "1.000 troy oz (31.103g)," and the coin is inscribed "1 OZ. FINE SILVER" (U.S. Mint bullion coin programs).

The gold coin is a useful example of why the wording matters. The same page lists the one-ounce American Eagle gold coin at "1.0909 troy oz. (33.931 g)" with a composition of "91.67% Gold 3% Silver Balance Copper." The Mint says these coins "contain their stated amount of pure gold, plus small amounts of alloy." So the coin's total weight is not the same as its gold content. The total weight includes the alloy. We did not open a source that spells out the gold weight in the other sizes, so we do not list them here.

The platinum and palladium coins on the same page are listed at "1.005 troy oz. (31.120 g)."

Why can mixing up ounces misprice a purchase?

We did not open a market source on how prices are quoted, so check the unit on every quote. Our guide to the gold spot price covers how quotes work. If you weigh metal in grams or in everyday ounces, you must convert before you compare prices.

Worked example (illustrative only). The price below is a round number made up for this example. It is not a market price and not a forecast.

Suppose a price of $1,000 per troy ounce, and 100 grams of metal.

Treating the troy-ounce price as if it were per everyday ounce overstates the value by about $312.32, or roughly 9.7%.

The takeaway is simple: check which ounce a quote, a scale or a product listing uses before comparing numbers.

What mistakes do people make with ounces?

• Weighing metal on a kitchen scale in ounces and applying a troy-ounce price.
• Assuming a coin's total weight equals its gold content. As the U.S. Mint page shows, alloy adds weight.
• Rounding too early. Rounding 31.1035 down to 31 grams changes the result by about 0.3% (31 divided by 31.1035), and the gap grows with larger amounts.
• Using different units in one comparison, such as grams from one seller and ounces from another.
• Forgetting that "pound" also has two versions. NIST lists a troy pound of 373.241 721 6 grams.

What related questions should you look at?

• Gold Spot Price Explained covers how quoted prices work.
• What Drives Gold Prices covers the factors we have sourced.
• If you are weighing dealer quotes for IRA metals, ask each seller in writing which unit its price uses and whether the weight shown is total weight or gold content.