What Is a Troy Ounce?

A troy ounce is the traditional unit for weighing gold and other precious metals, and it is larger than the ounce used for everyday goods. This guide explains where it comes from and how to convert it.

Two Different Ounces

English-speaking countries use two different ounces, and confusing them is one of the most common errors in gold calculations. The avoirdupois ounce, used for food, parcels, and most everyday weighing, equals about 28.35 grams. The troy ounce, used for gold, silver, and other precious metals, equals about 31.1035 grams. The troy ounce is therefore about ten percent heavier.

Whenever you see a gold price quoted per ounce, it means per troy ounce unless stated otherwise. Using the everyday ounce instead would understate the weight and overstate the price per gram, so getting this distinction right is essential for any gold calculation.

Where the Troy System Comes From

The troy system is named after Troyes, a market town in medieval France where merchants traded at famous fairs. The troy pound and troy ounce became standard units for precious metals and gemstones in England and spread through trade to much of the world. Although most countries have since adopted the metric system for everyday use, the troy ounce has survived in precious-metal markets because the entire global trading system is built around it.

Other troy units still appear occasionally: a troy pound contains twelve troy ounces, unlike the avoirdupois pound of sixteen ounces, and a pennyweight equals one twentieth of a troy ounce. For most readers, the troy ounce alone is the unit that matters, since that is how prices are quoted and how bars and coins are denominated.

Troy Ounces and Grams

The exact conversion is that one troy ounce equals 31.1034768 grams, usually rounded to 31.1035 grams for practical purposes. To convert a price per troy ounce to a price per gram, divide by 31.1035. To convert a weight in grams to troy ounces, divide the gram weight by 31.1035. To go the other way, multiply troy ounces by 31.1035 to get grams.

A kilogram of gold contains about 32.15 troy ounces, which is why a kilogram bar is slightly more than thirty-two ounces rather than a round number. Jewellers and dealers work in grams for small items, but the international price always traces back to the troy ounce, so this conversion sits at the heart of every local price calculation.

Common Gold Weights in Troy Ounces

Investment bars and coins are commonly sold in troy-ounce sizes: one ounce, half an ounce, a quarter ounce, and a tenth of an ounce are all standard. A one-ounce gold coin therefore contains one troy ounce of gold, or about 31.1 grams, if it is pure, and slightly more total weight if it is an alloy coin such as a 22-karat coin, where the gold content is one troy ounce but the coin also contains alloy metal.

Smaller bars are often sold in grams instead, particularly in Asia and Europe, where metric weights are more familiar. A ten-gram bar, a twenty-gram bar, and a hundred-gram bar are all common. Whatever the labelled weight, the value calculation is the same: find the fine gold content in grams, then multiply by the price per gram.

Why the Troy Ounce Persists

It would arguably be simpler if the whole world priced gold per gram, and many retail markets effectively do, by converting the troy-ounce price for local use. But changing the global benchmark would require every exchange, contract, and pricing system to change at once, and the costs and risks of such a switch have kept the troy ounce in place.

For the reader, the practical answer is simply to know the conversion and use it consistently. Once you remember 31.1035, you can move fluently between the international price and the gram prices used by jewellers, and you will never be caught out by the difference between the two ounces again.

Quick Reference

One troy ounce equals about 31.1035 grams. One gram equals about 0.03215 troy ounces. One kilogram equals about 32.1507 troy ounces. One avoirdupois ounce, the everyday ounce, equals about 28.3495 grams and is not used for gold.

Keep these figures handy when using any gold calculator. If a result looks surprising, the first thing to check is whether the calculation used troy ounces or everyday ounces, since that single error explains most puzzling gold valuations.