Troy Weight System
Also: Troy weight, Apothecaries' weight system, troy weight
The troy weight system is the internationally binding system of measures for precious metals, whose base unit is the troy ounce at exactly 31.1034768 grams.
The troy weight system is the oldest still-living system of measures in Western trade and remains to this day the only recognised weighing standard for gold, silver, platinum and all other precious metals. Whether the gold price is quoted in New York, London or Zurich - the unit is always the troy ounce (abbreviated: troy oz, oz t). Anyone who buys, sells or values precious metals cannot avoid the troy system.
Origin and History
The name is thought to derive from the northern French trading town of Troyes, which was an important commercial centre in the Middle Ages. There, merchants established a reliable weight system for the precious metal and coin trade. The Normans brought it to England, where it was fixed by statute for coinage in the 15th century. The British monetary system retained troy weight into the 20th century, while general commerce switched early on to the avoirdupois system - the system still used today for everyday goods in the USA.
Structure of the System
The troy system has only a few units but is internally fully coherent:
| 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 comprises 12 ounces - in contrast to the avoirdupois pound with 16 ounces. In modern precious metal trading, the troy pound is hardly used; the practically sole trading unit is the troy ounce.
The Crucial Risk of Confusion: Troy Ounce != Trade Ounce
The 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 = approx. +9.7% (troy is heavier)
Anyone who uses the wrong type of ounce in the melt value calculator or the unit converter obtains a material value distorted by almost 10%. For bars and coins, the troy ounce always applies.
Why No Switch to Grams?
Although the metric system is internationally dominant and many providers also display prices "per gram", all the major trading venues - LBMA, COMEX, the OTC market - quote in troy oz. The reasons are:
- Historical continuity: price series go back decades; a change would complicate all historical comparisons.
- International uniformity: from Perth to New York the same basis applies.
- Anchoring in standards: LBMA Good Delivery bars are specified in troy oz (350-430 troy oz, nominal weight approx. 400 oz).
Related Units in Other Cultural Regions
Alongside the troy system, regional units of weight exist that are still in use in their markets:
- Tola (South Asia): 11.6638 g = approx. 3/8 troy oz; a common bar denomination in India.
- Tael (East Asia): approx. 37.429 g (varies by region); widespread in Hong Kong and China.
- Momme (Japan): 3.75 g; traditionally used for jewellery.
In Brief
The troy weight system, with its core unit the troy ounce (31.1034768 g), is the indispensable reference framework of the global precious metal trade. Anyone who checks coins, values bars or compares prices always works in troy measure - confusing it with the avoirdupois ounce costs almost 10% in accuracy.