Definition
A system of weights historically used in law and commerce for measuring precious metals, gemstones, and apothecary compounds. The basic denominations are:
- 24 grains = 1 pennyweight
- 20 pennyweights = 1 ounce
- 12 ounces = 1 pound (troy)
The troy pound is lighter than the avoirdupois pound (which contains 16 ounces), though the troy ounce is heavier than the avoirdupois ounce. In legal contexts, troy weight appears most often in statutes and instruments governing coinage, bullion transactions, precious stone valuation, and, historically, pharmaceutical compounding.
Common Language
Modern common usage (Wiktionary): A system of units of weight or mass, mostly used for precious metals and gemstones, in which the pound contains twelve ounces which each contain 480 grains.
Historical common usage (Webster's 1913): Troy weight is the system in which 24 grains equal one pennyweight, 20 pennyweights equal one ounce, and 12 ounces equal one pound, used for gold, silver, and gems.
The common and legal definitions are largely aligned in substance, but the gap that matters for researchers is scope: common usage treats troy weight as a practical measuring convention, while legal usage embedded it in statutes, mint regulations, and commercial instruments where the specific denominations carried binding force. Misidentifying whether a historical contract or statute used troy or avoirdupois measurement can affect the interpretation of quantities by meaningful margins.
Common Confusion
Troy weight is easily conflated with avoirdupois weight, the system used for most everyday commercial goods, and with apothecary weight, a related but distinct system. Though apothecaries historically used the troy ounce (480 grains), they subdivided it differently: into 8 drams, each divided into 3 scruples, making one scruple equal to 20 grains. A legal instrument or statute specifying weight in "ounces" without qualification may refer to any of these systems depending on the subject matter and era. Precious metals and gemstones presumptively used troy weight; bulk commodities used avoirdupois; pharmaceutical compounding used apothecary weight derived from the troy base.
Why It Matters in Research
Researchers encounter troy weight most often in three contexts: coinage and mint law, commercial contracts for bullion or jewelry, and historical pharmaceutical regulation.
In coinage law, troy weight was the mandatory standard. U.S. mint statutes specified coin weights in troy grains and ounces, and discrepancies between troy and avoirdupois measurements were a live legal issue in bullion disputes.
In commercial instruments, the system used to define weight directly affects the quantity delivered or owed. A historical contract specifying pounds of silver is ambiguous without knowing whether troy or avoirdupois pounds were intended — a difference of four ounces per pound. Courts interpreting older instruments should be expected to look to trade usage, subject matter, and applicable statutes to resolve that ambiguity.
In pharmaceutical law, apothecary weight shared the troy ounce as its base but used different subdivisions (drams and scruples rather than pennyweights). Researchers examining drug compounding regulations, pharmacopoeial standards, or apothecary licensing disputes must distinguish between these systems even though both trace to the troy foundation.
Troy weight is largely obsolete as a legal standard for everyday commerce, having been displaced by metric and avoirdupois conventions, but it remains operative in precious metals trading, where the troy ounce continues as the standard unit, and in interpreting historical instruments.
Historical Dictionary Support
The historical dictionaries converge on the essential definition — 12 ounces to the pound, associated with gold, silver, and precious stones — but vary in what they add.
Burrill is the most useful of the shelf sources. He records the competing etymological claims (Troyes, France versus the monkish name "Troy Novant" for London, meaning "London weight") and notes that the system was chiefly used for gold, silver, diamonds, and jewelry. The London derivation is largely discredited by modern historians, but Burrill's recording of it reflects that the question was genuinely open in the period his dictionary addressed.
Bouvier provides the most practically useful breakdown, laying out the full denomination table and flagging the apothecary subdivision of the ounce into drams and scruples. This is the entry researchers should reach for when working through historical pharmaceutical or compounding disputes.
Black's (both editions) provides only the headline definition without subdivision detail. Anderson's simply cross-references COIN and WEIGHT, which is appropriate given the term's derivative nature but leaves the researcher doing additional work.
What the historical dictionaries collectively omit: any engagement with the statutory framework that gave troy weight its binding legal force, and any discussion of the jurisdictional transition away from troy weight as metrication and avoirdupois standardization proceeded. Researchers should supplement these entries with contemporaneous mint and weights-and-measures statutes for the jurisdiction and period in question.
Jurisdictional Note
In the United States, federal coinage law fixed troy weight as the standard for mint purposes from the early republic. In England, troy weight was formally abolished for most commercial purposes by the Weights and Measures Act 1878, which made avoirdupois the standard for trade, though troy measurement persisted in bullion and pharmaceutical contexts. Researchers working across jurisdictions or periods should confirm which weights-and-measures statute governed at the relevant time, as transitions were neither simultaneous nor complete.