Troy Weight System
Also: troy weight, apothecaries\' weight system, troy system
The troy weight system is the internationally binding system of measurement for precious metals, whose base unit is the troy ounce at exactly 31.1034768 grams.
The troy weight system is the oldest surviving system of measurement in Western trade and to this day the only recognised system of weights 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 (abbreviation: troy oz, oz t). Anyone who buys, sells, or appraises precious metals cannot avoid the troy system.
Origin and History
The name most likely derives from the northern French market town of Troyes, which was an important commercial centre in the Middle Ages. There, merchants established a reliable system of weights for precious metal and coin trading. The Normans brought it to England, where it was codified for coinage in the 15th century. The British monetary system retained troy weight into the 20th century, while general commodity trade had already switched to the avoirdupois system – the system still used for everyday goods in the United States.
Structure of the System
The troy system has only a few units but is internally fully coherent:
| Unit | Abbreviation | Grams | Relationship |
|---|---|---|---|
| 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 – in contrast to the avoirdupois pound with 16 ounces. In modern precious metal trading the troy pound is rarely used; the practically exclusive unit of trade is the troy ounce.
The Critical Risk of Confusion: Troy Ounce ≠ Avoirdupois 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 ≈ +9.7 % (troy is heavier)
Anyone using the wrong ounce in the melt value calculator or the unit converter will obtain a material value distorted by almost 10%. Bars and coins always state the troy ounce.
Why Not Switch to Grams?
Although the metric system is internationally dominant and many providers also display prices "per gram", all authoritative trading venues – LBMA, COMEX, OTC market – quote in troy oz. The reasons are:
- Historical continuity: Price series extend back decades; a switch would complicate all historical comparisons.
- International uniformity: From Perth to New York the same basis applies.
- Normative embedding: LBMA Good Delivery Bars are specified in troy oz (350–430 troy oz, nominal weight approx. 400 oz).
Related Units in Other Cultural Contexts
Alongside the troy system, regional units of weight that are still in common use in their respective markets exist:
- Tola (South Asia): 11.6638 g ≈ 3/8 troy oz; 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 frame of reference for global precious metal trading. Anyone checking coins, appraising bars, or comparing prices is always working in troy measure – confusing it with the avoirdupois ounce costs almost 10% in accuracy.