Why is bullion measured in troy ounces?
The unit you'll see on every gold and silver coin you own is over two thousand years old. Here's the story, from Roman bronze bars to medieval French market towns to the bar of bullion you can buy in Christchurch today.
The 30-second answer
- A troy ounce weighs 31.1035 grams, about 10% heavier than the regular "avoirdupois" ounce (28.3495 grams) you'd weigh groceries with.
- It traces back to the Roman uncia, a unit of weight from Rome's bronze currency system around the 3rd century BC. The Roman uncia weighed almost exactly the same as today's troy ounce.
- The name "troy" likely comes from Troyes, a French market town that hosted Europe's biggest medieval trade fairs in the 12th and 13th centuries.
- England's Henry II adopted it for coinage. By the 1500s it was Britain's official measure for gold and silver. The US followed in 1828.
- Why precious metals still use it: centuries of trust, contracts, and global standardisation. Every bullion price you see (LBMA, COMEX, Pure Bullion) is quoted per troy ounce.
The short version
When you buy a "1oz" gold coin, that's a troy ounce, not the same ounce you use for a steak or a bag of sugar. The reason gold and silver are weighed differently goes back to a system that's older than the English language. To understand it, we need to go back nearly 2,300 years to ancient Rome.
It all started with Rome
Before coins: bronze by weight
Long before Rome had a single round coin, Romans paid for things with lumps of bronze. Literally. They were called aes rude ("rough bronze") and you'd weigh them on a scale at the moment of trade. A loaf of bread? That'll be 50 grams of bronze. A goat? Several kilograms.
This was clunky, but it worked. And it tells you something important: from the very beginning, Roman money was about weight, not denomination. The weight was the value.
A side note that's too good to skip. The Latin word for money, pecunia, came from pecus, meaning cattle or sheep. Romans literally counted their wealth in livestock before they counted it in metal.
Bronze bars: the first stamped currency
Around the 4th century BC, Romans started casting bronze into flat bars stamped with images: eagles, bulls, ships, anchors. These were called aes signatum ("signed bronze"), and they typically weighed about 1.5 to 1.6 kilograms each, or roughly five Roman pounds.
These weren't quite coins yet. There was no fixed denomination. But they were the first form of standardised, recognisable Roman currency. Trade got easier. Disputes got fewer.
The libra and the uncia
In 269 BC, Rome introduced its first proper coin system, called aes grave ("heavy bronze"). The base coin was the as, which weighed exactly one Roman pound (libra), about 324 grams.
Here's the part that matters for our story:
The word uncia literally meant "one-twelfth." It's where we get both the modern words "ounce" and "inch" (a twelfth of a foot). The Roman libra is also where Britain's pound symbol comes from. That's why £ has a horizontal line through an "L."
Even today, when a Kiwi sees "lb" written on a parcel, they're looking at an abbreviation for libra, Latin for pound, sitting on the desk of every accountant in the world, untouched for 2,300 years.
The denarius: the coin that ruled Europe for 500 years
Around 211 BC, Rome introduced a small silver coin that would become the most important currency in the ancient world: the denarius.
The denarius weighed about 4.5 grams of silver, roughly 1/72nd of a Roman pound. Its name came from deni, meaning "ten," because it was originally worth 10 bronze asses.
This little silver coin became the workhorse of the Roman economy for nearly five centuries. It paid soldiers, bought bread, and circulated from Britain to Egypt. When Jesus said "render unto Caesar," the coin in question was almost certainly a denarius.
The denarius established something profound: that precious metal coins should have a fixed weight, certified by an authority, recognised across the empire. That principle is exactly what every modern gold coin still does, 2,200 years later.
The denarius's legacy is everywhere:
- The British pre-decimal penny was abbreviated "d", for denarius. Right up until 1971.
- The dinar (still used in Jordan, Iraq, and elsewhere) descends directly from it.
- The Spanish dinero and Portuguese dinheiro (meaning "money") are both from denarius.
Rome falls. The weights survive.
The Western Roman Empire collapsed in 476 AD. But here's the strange and beautiful thing: the weights didn't die with it.
Across medieval Europe, in monasteries, royal mints, market towns, the Roman pound and ounce kept being used for one specific purpose: measuring precious metals. Even as kingdoms rose and fell, every silversmith and goldsmith on the continent still weighed their gold by some descendant of the uncia.
This is the bridge from Rome to Troyes.
The medieval fairs of Troyes, France
By the 12th and 13th centuries, the most important trading fairs in Europe weren't in Rome or London. They were in a town in northeastern France called Troyes, in the Champagne region.
These fairs were huge. Twice a year, merchants from across Europe (Italian silver dealers, Flemish cloth traders, English wool merchants, Arab gold buyers, Jewish moneylenders) would converge on Troyes to buy and sell. It was the medieval equivalent of a global trade summit.
And there was a problem.
Why Troyes needed its own ounce
Every region of Europe had a slightly different "ounce." A pound in London wasn't the same as a pound in Cologne, which wasn't the same as a pound in Paris. When you're trading gold and silver, even a 5% difference in weight could ruin a deal.
The merchants in Troyes did something brilliant: they agreed on one standardised weight that everyone would use for precious metals at the fair. That weight became known as the "troy" ounce, named after the city.
What's remarkable is that the troy ounce of medieval Troyes ended up being almost identical to the Roman uncia of 1,200 years earlier, about 31.1 grams.
Some historians believe this wasn't coincidence: the merchants of Troyes likely modelled their system on the surviving Roman tradition, which had never really gone away in monastic and minting circles.
The Roman uncia, the medieval troy ounce, and the modern troy ounce are essentially the same weight. One unit, two thousand years, three civilisations.
England picks it up and locks it in
England adopted the troy ounce for coinage during the reign of Henry II (1154 to 1189). By the time of Henry VIII in 1527, troy weight was the official standard for gold and silver in Britain, and Britain was rapidly becoming the centre of global bullion trading.
The system the English locked in was beautifully precise:
- 1 grain (originally based on a barleycorn) = 0.0648 g
- 24 grains = 1 pennyweight (abbreviated "dwt", d for denarius!)
- 20 pennyweights = 1 troy ounce = 480 grains = 31.1035 grams
- 12 troy ounces = 1 troy pound = 373.24 grams
You'll notice 240 pennyweights = 1 troy pound, and there were also 240 pennies in the original British pound sterling. That's not a coincidence either. The whole system was designed to mesh together.
When London became the centre of world bullion trading in the 1700s and 1800s (eventually formalised as the London Bullion Market Association, the LBMA) the troy ounce became the global standard. It hasn't moved since.
The United States adopted it in 1828 for the regulation of US coinage. New Zealand inherited it from Britain. Today, every bullion exchange on Earth (London, New York, Shanghai, Sydney) quotes precious metals in troy ounces.
So where did the "regular" ounce come from?
You're probably wondering: if everyone agreed on the troy ounce, why do we have a different ounce for everyday things?
The answer is that two systems evolved side by side.
The everyday ounce is called the avoirdupois ounce, from Old French avoir de pois, meaning "goods of weight." It was developed for bulk commodities: wool, grain, spices, meat. Things sold by the wagon-load, where extreme precision didn't matter.
The avoirdupois ounce weighs 28.3495 grams, about 10% lighter than a troy ounce.
So we ended up with two ounces in parallel use:
- Troy ounce (31.1 g) for precious metals, gemstones, coins.
- Avoirdupois ounce (28.35 g) for groceries, bodies, packages, everything else.
Confusingly, this also means a troy pound (12 troy ounces = 373 g) is actually lighter than an avoirdupois pound (16 avoirdupois ounces = 454 g). Same word, different weight, depending on what you're weighing.
This is why you should never trust someone who quotes you a "1oz gold coin price" without specifying which ounce they mean. A 10% gap on the wrong side can cost you serious money.
Quick reference: troy vs avoirdupois
| Unit | Troy | Avoirdupois |
|---|---|---|
| 1 ounce | 31.1035 g | 28.3495 g |
| Used for | Gold, silver, platinum, palladium | Groceries, postage, body weight, bulk goods |
| Grains per ounce | 480 | 437.5 |
| Ounces per pound | 12 | 16 |
| Pound weight | 373.24 g | 453.59 g |
| Origin | Roman uncia, medieval Troyes | Old French avoir de pois |
A simple rule: a troy ounce is heavier than a regular ounce by about 10%. So if you ever see "1oz gold" priced at the avoirdupois price, you're being short-changed by about 10%. Always look for the marking "oz t", "ozt", or "troy oz" on bullion products.
Why bullion still uses troy in 2026
You'd think we'd have switched to grams by now, right? Most countries did. New Zealand officially uses the metric system. So why does every gold coin still say "1 oz"?
A few reasons:
- Centuries of contracts. Every bullion contract, futures market, and spot price reference dating back hundreds of years is in troy ounces. Switching would create chaos.
- Global standardisation. The LBMA in London, COMEX in New York, the Shanghai Gold Exchange. They all clear contracts in troy ounces. Switching unilaterally would isolate any country that did it.
- Coin tradition. The American Eagle, Canadian Maple Leaf, Krugerrand, Gold Kiwi, Britannia, Perth Mint Kangaroo. Every major bullion coin is minted to one troy ounce of pure metal. That's a 200+ year manufacturing tradition.
- Investor familiarity. "An ounce of gold" is something everyone understands. Talking in grams ($150 per gram?) just feels alien to most buyers.
Some markets (India, the UAE, China) do quote gold per gram. But when those prices feed into global benchmarks, they're converted back to troy ounces. The troy ounce has won so completely that even the gram-based markets have to deal with it.
What this means when you buy gold or silver in NZ
If you're buying bullion in Christchurch, Auckland, Wellington, anywhere in New Zealand, here's what to watch:
- All NZ bullion products are in troy ounces. A "1oz silver coin" is one troy ounce (31.1 grams), not 28.3 grams. Same for "10 oz silver bars," 311 grams of silver.
- NZD spot prices are quoted per troy ounce. When you see "gold at NZ$7,880/oz" anywhere, that's per troy ounce.
- Convert to grams to compare with international markets. Divide by 31.1035. Useful when checking whether you're getting a fair NZ price compared with overseas sellers.
- Watch out for jewellery. Some jewellers quote gold prices per gram, or per "pennyweight" (1.555 g). That's fine, just convert before comparing.
- Know what you're buying. A gold coin's gross weight (the whole coin) is slightly heavier than its fine weight (the actual gold) because most coins are alloyed with copper or silver for durability. A "1oz Gold Eagle" contains 1 troy oz of pure gold but the whole coin weighs 1.0909 troy oz.
Buying gold and silver in Christchurch
Now you know why your gold coin says "1oz," and what that "oz" actually means.
When you're ready to start (or grow) your stack, Pure Bullion will soon be stocking gold and silver bullion for Kiwi buyers, with transparent NZD pricing per troy ounce, exactly the way it should be.
Whether you're searching for "buy gold Christchurch," "silver bullion NZ," or just want to own a piece of the same weight system Roman senators and medieval merchants used, keep an eye on Pure Bullion.
Local. Trusted. Coming soon.
Frequently asked questions
Why is gold measured in troy ounces?
Gold and other precious metals are measured in troy ounces because the system traces back over 2,000 years to the Roman uncia, was standardised at the medieval trade fairs of Troyes in France, and was formally adopted by the British Crown for coinage. By the time the modern global bullion market took shape in 1700s London, the troy ounce was already the entrenched standard. Today every major bullion exchange (LBMA, COMEX) quotes precious metals exclusively in troy ounces.
How much is a troy ounce in grams?
One troy ounce equals 31.1035 grams. To convert any troy-ounce price to per-gram, divide by 31.1035.
What's the difference between a troy ounce and a regular ounce?
A troy ounce weighs 31.1035 grams, while a regular (avoirdupois) ounce weighs 28.3495 grams, meaning a troy ounce is approximately 10% heavier. The troy ounce is used for precious metals; the avoirdupois ounce is used for groceries, postage, and most everyday items.
Where does the word "ounce" come from?
"Ounce" comes from the Latin uncia, meaning "one-twelfth." In ancient Rome, an uncia was 1/12 of a Roman pound (libra), weighing about 27 to 31 grams. The same Latin root also gave us the word "inch", one-twelfth of a foot.
Why is it called a "troy" ounce?
The name most likely comes from the city of Troyes in northeastern France, which hosted some of medieval Europe's most important trading fairs in the 12th and 13th centuries. Merchants from across the continent agreed on a standardised weight for precious metals there, and the unit became known as the "troy" ounce.
What was the main currency of ancient Rome?
The denarius was the main silver coin of Rome for nearly 500 years (from 211 BC to the 3rd century AD). It originally weighed about 4.5 grams of nearly pure silver, roughly 1/72nd of a Roman pound. The denarius's name lives on in the modern dinar (Middle East), the Spanish word "dinero," and the abbreviation "d" used for the British penny until 1971.
Did Romans really pay for things with bronze bars?
Yes. Before round coins, Romans used cast bronze bars called aes signatum, typically 1.5 to 1.6 kg each, stamped with images like eagles, bulls, ships, and anchors. These were the precursor to formal coinage and circulated from around the 4th to mid-3rd century BC.
Is the troy ounce used everywhere?
Almost. The troy ounce is the standard for precious metals in every major bullion market: London (LBMA), New York (COMEX), Sydney, Singapore, Shanghai, Tokyo, Zurich. Some retail markets, especially in India, the UAE, and parts of China, quote gold per gram for everyday consumers, but those prices still trace back to troy-ounce-based global benchmarks.
How do I convert troy ounces to grams?
Multiply troy ounces by 31.1035 to get grams. Divide grams by 31.1035 to get troy ounces. Example: a 10oz silver bar = 311.035 grams. A 100g gold bar = 3.215 troy ounces.
Does New Zealand use troy ounces for bullion?
Yes. All bullion sold in New Zealand (coins, bars, rounds) is measured in troy ounces, and NZD spot prices for gold and silver are quoted per troy ounce. This matches the international standard.
The bottom line
When you hold a one-ounce gold coin, you're holding a piece of weight that connects you directly to a Roman senator weighing out a denarius, a medieval merchant in a French market town, and Henry VIII reforming English coinage in 1527.
The troy ounce isn't an arbitrary measurement. It's a 2,300-year-old standard that survived the fall of empires, the rise of nations, and the entire conversion to the metric system, because once humanity finds a unit that works for measuring real wealth, it doesn't let go.
That's the unit your gold coin uses. That's the unit you'll see on every Pure Bullion product. And now you know why.
Want more bullion fundamentals? Read our beginner's guide to bullion in New Zealand, our comparison of gold, silver and copper, or the latest monthly market update.
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Pure Bullion publishes monthly market updates, beginner guides, and educational pieces to help Kiwis understand and invest in precious metals. This article is for general information only and does not constitute financial advice.