Bullion is not weighed in the ounces you use in the kitchen. Precious metals use the troy ounce, a separate and older unit, and the difference is large enough to matter.
The number
A troy ounce is 31.1035 grams. A standard avoirdupois ounce, the one used for food and post, is 28.3495 grams.
A troy ounce is therefore about ten percent heavier. When a bullion product says one ounce, it means 31.1035 grams, and that is what you are paying for.
Where the unit comes from
The troy system takes its name from Troyes in France, a major medieval trading centre, and it has been used for precious metals for centuries. It survived the shift to metric in most countries specifically because global metal markets were already standardised on it, and changing would have been more disruptive than keeping it.
Spot prices for gold, silver and platinum are all quoted per troy ounce.
The troy pound trap
Here is the part that catches people out. A troy pound contains 12 troy ounces, not 16.
So although a troy ounce is heavier than a standard ounce, a troy pound is lighter than a standard pound, because it contains four fewer ounces. Troy pounds are rarely used in modern bullion trading, but the inconsistency is worth knowing if you encounter older references.
Grams, kilos and common sizes
Most bullion is sold either in troy ounces or in straight metric weights, and both appear side by side.
Common gold sizes run from one gram through 5g, 10g, 20g, 1oz, 100g and 1kg. Common silver sizes run from 1oz through 5oz, 10oz, 500g and 1kg.
Useful conversions: one troy ounce is 31.1035 grams, so a 100 gram bar is roughly 3.215 troy ounces, and a one kilogram bar is roughly 32.15 troy ounces.
Reading fineness
Weight tells you how much metal. Fineness tells you how much of that metal is the metal you want.
Fineness is expressed in parts per thousand. A marking of 999 means 999 parts per thousand, or 99.9 percent. A marking of 9999 means 99.99 percent. Carat is a different scale used mainly for gold, where 24 carat is the top of the scale and 22 carat is 91.67 percent gold.
The two systems are not interchangeable shorthand, and a coin can be described either way, so read the specification rather than assuming.
Gross weight and fine weight
One more distinction. Gross weight is what the item weighs in total. Fine weight is how much pure precious metal it contains.
For a 9999 bar the two are almost identical. For an alloyed coin they are not. A 22 carat coin containing one troy ounce of gold has a gross weight noticeably above one troy ounce, because of the alloy metal added for hardness. When comparing products, compare fine weight.
Frequently asked questions
How many grams in a troy ounce?
31.1035 grams.
Is a troy ounce the same as a normal ounce?
No. A troy ounce is 31.1035 grams and a standard ounce is 28.3495 grams, so a troy ounce is about ten percent heavier.
How many troy ounces in a kilogram?
Approximately 32.15 troy ounces.
What does 999 fine mean?
999 parts per thousand of the stated metal, or 99.9 percent purity.
What is the difference between gross weight and fine weight?
Gross weight is the total weight of the item. Fine weight is the weight of pure precious metal it contains. They differ for alloyed products.
Browse by weight
We stock bullion from one gram upward across gold, silver, platinum, copper and titanium, with weight and fineness stated on every listing. See the range or visit the showroom at 124 Peterborough Street, Christchurch.