Two ounces. One word. A 9.7% gap.
The word "ounce" names two different weights that share a common Latin ancestor — uncia, meaning one-twelfth — but diverged in the 14th century and never reconciled. The avoirdupois ounce (exactly 28.349523125 g since 1959) is the ounce of commerce: food labels, postal scales, kitchen recipes, body weight. The troy ounce (31.1034768 g) is the ounce of precious metals: gold bars, silver coins, platinum futures. The troy ounce is 9.7% heavier. A one-ounce gold coin weighs 31.1 g; a one-ounce bag of chips weighs 28.3 g. Same word on the label. Different gram count on the scale.
This is not a rounding convention or a regional variation. The avoirdupois and troy systems are entirely separate unit hierarchies that collided in the word "ounce." The avoirdupois pound = 16 avoirdupois ounces = 7,000 grains = 453.59237 g. The troy pound = 12 troy ounces = 5,760 grains = 373.24172 g. They agree only on the grain: 1 grain = 64.79891 mg exactly in both systems. Everything above the grain diverges.
The troy ounce: a medieval French coinage standard that never died
The troy weight system takes its name from Troyes, a market town in northeastern France that hosted major trade fairs in the Middle Ages. Merchants from across Europe converged there, and the troy ounce — roughly the weight of a silver penny — became the de facto standard for coinage and precious metals. The English mint adopted it formally in 1527 under Henry VIII, and the United States inherited it via British coinage practice. The US Mint still strikes gold and silver coins in troy ounces.
Unlike the avoirdupois ounce, which was redefined in exact metric terms by the 1959 agreement, the troy ounce was never legally tied to a metric definition by international treaty. Its value — 31.1034768 g — derives from the grain (1 troy ounce = 480 grains) and the grain's exact metric equivalent (1 grain = 64.79891 mg). That grain definition was adopted by the US, UK, and Canada in 1959 as part of the same agreement that fixed the avoirdupois pound. So both ounces trace to the same grain, but they count different numbers of grains: 437.5 grains per avoirdupois ounce, 480 grains per troy ounce.
Where the wrong ounce costs real money
- Gold and silver retail. A "1 oz" gold coin is a troy ounce — 31.1035 g. If a buyer weighing it on a kitchen scale reads 28.35 g and thinks they've been shorted, they are using the wrong ounce. If a seller advertises "1 oz of gold" at the avoirdupois weight (28.35 g), they are under-delivering by 9.7% and committing fraud in most jurisdictions.
- Precious metal ETFs and futures. Gold futures on COMEX are specified in troy ounces. Every institutional trade settles in troy ounces. An algorithm that accidentally converts using avoirdupois ounces misprices the contract by 9.7% — on a single gold futures contract (100 troy oz at $2,000/oz), that's a $19,400 error.
- Pharmaceutical compounding. Historical pharmacopoeias sometimes used troy ounces for active ingredients. A modern pharmacy reconstructing a historical formulation must know which ounce the original used — or risk a 9.7% overdose or underdose.
- Jewelry and gemstones. Diamond weight is in carats (0.2 g), but gold settings are sold by the troy ounce. Pearl weight is sometimes stated in troy grains. A piece of jewelry can involve three different mass systems in a single sale.
The avoirdupois ounce: how the everyday ounce got its number
The avoirdupois system originated in 14th-century England during the reign of Edward III. The word itself is Old French — aveir de peis, "goods of weight" — for the weighing of bulk commodities like wool, grain, and metals. The system standardized the pound at 7,000 grains, divided into 16 ounces of 437.5 grains each. This 16-ounce pound was more convenient for commerce than the 12-ounce troy pound because it divided more naturally into halves, quarters, eighths, and sixteenths — the same reason a foot has 12 inches.
When the 1959 International Yard and Pound Agreement defined 1 avoirdupois pound = 0.45359237 kg exactly, the avoirdupois ounce inherited an exact metric value for the first time in history: 28.349523125 g. Before 1959, the ounce was defined by the grain; after 1959, it was defined by the kilogram via the pound. The troy ounce, still defined by the grain (480 grains × 64.79891 mg/grain = 31.1034768 g), remained unchanged — the only common unit to survive the 1959 treaty without a direct metric definition.
Three worked conversions — each one tells you which ounce to use
Example 1 — recipe conversion (avoirdupois): 8 oz of flour × 28.3495 = 226.8 g. This is the ounce on every American measuring cup and every kitchen scale sold in the US. Never use troy for food — the 9.7% extra flour would ruin the dough.
Example 2 — gold investment (troy): 1 oz of gold × 31.1035 = 31.10 g. This is the ounce stamped on every gold bar, every gold coin, and every COMEX futures contract. At $2,000/oz, the difference between avoirdupois and troy is $194 per ounce — enough to get sued.
Example 3 — postage (avoirdupois): 3.5 oz letter × 28.3495 = 99.2 g. USPS and every other postal service in the world use avoirdupois ounces for weight-based postage. The troy ounce has never been used for mail — if your scale has a "troy" mode, make sure it's off before you weigh a package.
Ounce-to-gram reference — both systems
| Ounces | Grams (avoirdupois) | Grams (troy) | Which to use |
|---|---|---|---|
| 0.5 oz | 14.17 g | 15.55 g | Food, postage, everyday |
| 1 oz | 28.35 g | 31.10 g | Avoirdupois for retail; troy for gold |
| 2 oz | 56.70 g | 62.21 g | Kitchen vs. bullion |
| 4 oz | 113.40 g | 124.41 g | Quarter-pound vs. 4 troy oz bar |
| 8 oz | 226.80 g | 248.83 g | Half-pound of butter vs. 8 oz gold |
| 12 oz | 340.19 g | 373.24 g | 12 oz soda can vs. troy pound |
| 16 oz | 453.59 g | 497.66 g | Avoirdupois pound vs. 16 troy oz |
| 32 oz | 907.18 g | 995.31 g | 2 lb bag vs. kilo bar (32.15 troy oz) |
Frequently asked questions
How do I convert ounces to grams?
For everyday items (food, postage, body weight): multiply ounces by 28.3495 to get grams. For precious metals (gold, silver, platinum): multiply by 31.1035. The two differ by 9.7%. Use the calculator at the top of this page — it defaults to the avoirdupois ounce. If you are holding gold, use 31.1035 instead and check your contract.
What is the difference between an avoirdupois ounce and a troy ounce?
The avoirdupois ounce (28.3495 g) divides the pound into 16 parts and is used for commerce. The troy ounce (31.1035 g) divides the troy pound into 12 parts and is used only for precious metals. They share the same grain (64.79891 mg) but count different numbers of grains: 437.5 per avoirdupois oz, 480 per troy oz. Using the wrong ounce in a gold transaction creates a 9.7% valuation error — actionable as fraud in most jurisdictions.
Why does the word 'ounce' mean two different things?
Both derive from the Latin uncia (one-twelfth), but they evolved in separate trades: commerce (avoirdupois, 14th century England) and minting (troy, medieval French coinage via Troyes). The English mint adopted the troy ounce in 1527; the avoirdupois ounce was standardized slightly later for general commerce. The two systems were never unified. The 1959 International Yard and Pound Agreement fixed the avoirdupois ounce in metric terms; the troy ounce remained tied to the grain, which was also fixed in metric terms by the same agreement — but with different arithmetic.
If a food package says '8 oz,' is that avoirdupois or troy?
Always avoirdupois. Food labeling in every jurisdiction uses avoirdupois ounces — 28.35 g per ounce. An 8 oz package weighs 226.8 g. If someone used troy ounces on a food label, the package would weigh 248.8 g — a 9.7% overfill that would violate FDA net-weight regulations (21 CFR 101.105). The troy ounce appears only on precious metals, and it is always explicitly labelled as "troy ounce" or "oz t" in regulated markets.
How do I convert ounces to grams for a recipe?
Use avoirdupois: 1 oz = 28.35 g. For most home cooking, rounding to 28 g is fine (1.2% error, smaller than a typical kitchen scale's accuracy). For baking, use the full 28.35 g — at scale, the difference affects hydration ratios. If the recipe is historical (pre-1850 British), check whether it uses troy — some old apothecary and spice recipes did. If in doubt, the smaller number (avoirdupois) is almost certainly correct.
Engineering context
Ounces-to-grams is the only common mass conversion that branches — and the branch point is invisible on the scale. The avoirdupois ounce (28.349523125 g exactly, by the 1959 treaty) covers virtually all non-precious applications. The troy ounce (31.1034768 g, by the grain definition) governs precious metals exclusively. The discipline is to verify which ounce you are converting before you multiply — a kitchen scale cannot do this, a gold scale defaults to troy and cannot do otherwise, and a general-purpose converter must make the distinction explicit. An engineer specifying material weights in a cross-border contract must state "ounce (avoirdupois)" or "ounce (troy)" — the word "ounce" alone is ambiguous and, in a legal context, the ambiguity runs against the drafter. For the reverse direction, see grams to ounces; for the abbreviated-form page, oz to grams; for the full weight-system treatment, refer to the Weight & Mass conversion guide.
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