You’re looking at a gold coin. It’s shiny, heavy, and feels like real money. You check the weight. It says one ounce. You throw it on your kitchen scale, the one you use for sourdough or mail, and it reads 31.1 grams. Wait. If you grew up in the US, you know an ounce is 28.35 grams. Did you get scammed? Is the scale broken?
Actually, no. You just bumped into the weird, stubborn world of precious metals measurement.
The truth is that there are exactly 31.1034768 grams in a troy ounce.
Most people have no clue that the "ounce" they use for sugar or steak isn't the same "ounce" used for gold, silver, or platinum. It’s a mess. If you use the wrong one while buying bullion, you're literally losing about 10% of your value instantly. That's a massive mistake when gold is trading at thousands of dollars per ounce.
Why the Troy Ounce Refuses to Die
History is messy. While most of the world moved to the metric system, and the US stuck with the British Imperial system (Avoirdupois), the precious metals market stayed stuck in the Middle Ages. Specifically, it stayed stuck in Troyes, France.
Back in the 12th century, the fair at Troyes was the place to be for international trade. Merchants needed a standardized weight for gold and silver. They settled on a system that eventually became the troy ounce. It’s been the global standard for nearly a millennium. Even when the French Revolution tried to kill off old units with the metric system, the troy ounce survived.
Why? Because the banking systems in London and New York were already built on it. Changing the weight of every central bank gold bar in the world would have been a logistical nightmare. So, we kept it.
The Avoirdupois ounce—the 28.35-gram one—is based on a pound of 16 ounces. The troy ounce is part of a troy pound, which only has 12 ounces. To make it even more confusing, a troy ounce is actually heavier than a standard ounce, but a troy pound is lighter than a standard pound. It's enough to give anyone a headache. Honestly, just stick to the grams.
The Math You Actually Need
Let’s get precise. If you are selling scrap jewelry or buying a silver round, you need the decimal.
One troy ounce is $31.103$ grams. Most dealers will round this to $31.1$ for quick math. If you’re dealing with massive quantities, like a 400-ounce "Good Delivery" bar held by the Federal Reserve, those tiny decimals start to represent thousands of dollars.
Think about it this way:
If gold is $2,500 per ounce, every single gram is worth about $80.38. If you accidentally calculate using 28.35 grams instead of 31.1, you are "missing" about 2.75 grams. That is a $221 error.
You wouldn't leave $200 on the table at a car dealership, so don't do it here.
Dealing with "DWT" or Pennyweights
Sometimes, especially in the world of vintage jewelry or dental gold, you’ll see the term "pennyweight" or DWT. This is another relic. There are exactly 20 pennyweights in a troy ounce.
- One troy ounce = 31.1 grams.
- One pennyweight = 1.555 grams.
If a jeweler tells you a ring weighs 10 pennyweights, it’s about 15.5 grams, or exactly half a troy ounce. It's a shorthand that old-school smiths still love because it makes the math easy when you're mixing alloys.
Buying Gold Without Getting Ripped Off
When you walk into a coin shop or browse an online bullion dealer like APMEX or JM Bullion, the prices are always listed per troy ounce. However, the physical products vary wildly.
A standard 1 oz Gold American Eagle is actually heavier than 31.1 grams.
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Wait, what?
It’s because the coin isn't pure gold. It’s 22-karat. To ensure it contains exactly one troy ounce of pure gold, the US Mint adds silver and copper to make the coin more durable. The total weight of the coin is actually about 33.93 grams. But the gold content? Exactly 31.1 grams.
On the flip side, a 24-karat coin like a Canadian Maple Leaf or a Buffalo will weigh exactly 31.1 grams because it is pure.
Common Pitfalls for New Investors
- The Kitchen Scale Trap: Most digital kitchen scales have a "mode" button. It usually toggles between grams (g), ounces (oz), and sometimes troy ounces (ozt). If your scale says "oz," it is likely calculating the 28.35-gram version. Always use the gram setting for accuracy.
- Sterling Silver Confusion: Sterling silver is 92.5% pure. If you have 100 grams of sterling silverware, you don't have 3.2 troy ounces of silver. You have about 2.97 troy ounces of actual silver content after you account for the copper alloy.
- The "Ounce" Label: If a listing on eBay just says "1 oz silver bar," verify it's troy. If it's a "copper ounce," those are almost always 28.35 grams. Copper isn't a precious metal in the same trading sense, so sellers often use the cheaper, lighter standard.
The Global Perspective
In many parts of Asia, particularly China and India, the troy ounce isn't the primary way people think about gold. You'll hear about the Tola in India (roughly 11.66 grams) or the Tael in Hong Kong and China (about 37.4 grams).
However, even in those markets, the international "spot price" that flashes on the news is based on the troy ounce. It is the lingua franca of wealth.
If you are traveling and want to buy gold in Dubai or Mumbai, take a calculator. You have to convert their local units back to grams, then divide by 31.1 to see if the price they are quoting you actually beats the New York or London spot price.
Actionable Steps for the Smart Buyer
If you’re serious about holding physical metal, stop thinking in ounces entirely for a moment. Think in grams. It’s the only way to be 100% sure across all borders and scales.
First, buy a high-quality milligram scale. A cheap scale that only goes to one decimal point (0.1g) isn't enough. You want one that goes to 0.01g. This allows you to see the difference between a real coin and a counterfeit that might be weighted with tungsten. Tungsten has a similar density to gold, but it's hard to get the weight and dimensions exactly right.
Second, always check the "ozt" vs "oz" setting. If you are using a digital scale, look for that tiny "t" next to the "oz." If it isn't there, you aren't measuring troy ounces. Switch to grams and look for that 31.1 target.
Third, know your purity.
- .999 fine = Multiply total weight by 1.
- .925 (Sterling) = Multiply total weight by 0.925.
- 14k Gold = Multiply total weight by 0.583.
Once you have the "pure" gram weight, divide it by 31.103. That is your true troy ounce count. Compare that to the current spot price, and you’ll know exactly how much "premium" you are paying over the market rate.
Understanding the difference between grams in a troy ounce and a standard ounce is the literal price of entry for the precious metals market. It’s a quirk of history that costs uninformed people money every single day. Don't let a 12th-century French measurement system catch you off guard.