Troy Weight System
Also: troy weight, apothecaries' weight system
The troy weight system is the internationally binding system of measures for precious metals, whose base unit is the troy ounce of exactly 31.1034768 grams.
The troy weight system is the oldest system of measures still in use in Western commerce and remains the only recognised system for weighing 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 fair town of Troyes, an important trading centre in the Middle Ages. There, merchants established a reliable weight system for precious-metal and coin trading. The Normans brought it to England, where it was fixed by statute for coinage in the 15th century. The British coinage system kept troy weight into the 20th century, while general commerce switched early on to the avoirdupois system — the system still used 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 grain |
| Troy ounce | troy oz | 31.1034768 g | 20 dwt / 480 grain |
| Troy pound | lb t | 373.2417 g | 12 troy ounces |
The troy pound comprises 12 ounces — in contrast to the avoirdupois pound of 16 ounces. In the modern precious-metals trade the troy pound is hardly used; the practically sole trading unit is the troy ounce.
The crucial risk of confusion: troy ounce ≠ commercial 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 kind of ounce in the melt value calculator or the unit converter will get a material value distorted by almost 10%. For bars and coins, the troy ounce always applies.
Why not switch to grams?
Although the metric system is internationally dominant and many providers also show prices "per gram", all the leading trading venues — LBMA, COMEX, the OTC market — quote in troy oz. The reasons are:
- Historical continuity: price series go back decades; a switch would complicate all historical comparisons.
- International uniformity: the same basis applies from Perth to New York.
- 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 there are regional units of weight still in use in their markets:
- Tola (South Asia): 11.6638 g ≈ 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-metals trade. Anyone checking coins, valuing bars or comparing prices always works in troy measure — confusing it with the avoirdupois ounce costs almost 10% in accuracy.