Troy Weight System
Also: Troy weight, Apothecaries' weight, troy system
The troy weight system is the internationally binding scale for precious metals; its base unit, the troy ounce, equals exactly 31.1034768 grams.
The troy weight system is the oldest surviving measurement standard of Western commerce and remains, to this day, the only recognised weighing scale for gold, silver, platinum and every other precious metal. Whether the gold price is quoted in New York, London or Zurich, the unit is always the troy ounce (abbreviated troy oz or oz t). Nobody who buys, sells or values precious metals can escape it.
Origins and history
The name is thought to come from Troyes, a market town in northern France that was a major trading hub in the Middle Ages. Merchants there settled on a dependable weighing standard for the metal and coin trade. The Normans carried it across to England, where by the fifteenth century it had been fixed in law for coinage. Britain's coinage system stuck with troy weight well into the twentieth century, even as everyday trade in goods had long since moved to the avoirdupois system - the very system still used for ordinary items in the United States today.
How the system is built
The troy system has only a handful of units, but internally it is perfectly consistent:
| Unit | Abbreviation | Grams | Ratio |
|---|---|---|---|
| Grain | gr | 0.06479891 g | smallest unit |
| Pennyweight | dwt | 1.55517 g | 24 grains |
| Troy ounce | troy oz | 31.1034768 g | 20 dwt / 480 grains |
| Troy pound | lb t | 373.2417 g | 12 troy ounces |
The troy pound contains 12 ounces - unlike the avoirdupois pound, which holds 16. In modern bullion trading the troy pound is almost never used; the troy ounce is in practice the sole unit of trade.
The crucial trap: troy ounce is not the everyday ounce
The single most common mistake when buying precious metals is confusing the troy ounce with the ordinary avoirdupois ounce:
1 troy oz = 31.1034768 g
1 avdp. oz = 28.3495 g
Difference ~ +9.7% (troy is heavier)
Feed the wrong kind of ounce into a melt value calculator or a unit converter and the material value that comes out will be off by nearly 10%. For bars and coins the troy ounce is always the one that applies.
Why not simply switch to grams?
Even though the metric system dominates internationally and many sellers now quote prices "per gram" as well, every authoritative trading venue - the LBMA, the COMEX and the OTC market - still quotes in troy ounces. The reasons are:
- Historical continuity: price series stretch back decades, and a switch would muddy every historical comparison.
- International uniformity: from Perth to New York the same base applies.
- Anchored in standards: LBMA Good Delivery bars are specified in troy ounces (350-430 troy oz, nominal weight around 400 oz).
Related units in other regions
Alongside the troy system, several regional weight units remain current in their home markets:
- Tola (South Asia): 11.6638 g, roughly 3/8 of a troy ounce; a common bar size in India.
- Tael (East Asia): about 37.429 g (it varies by region); widely used in Hong Kong and China.
- Momme (Japan): 3.75 g; traditional for jewellery.
In a nutshell
The troy weight system, with its core unit of the troy ounce (31.1034768 g), is the indispensable frame of reference for the global precious metal trade. Anyone checking coins, valuing bars or comparing prices is working in troy measure - and mixing it up with the avoirdupois ounce costs almost 10% in accuracy.