What is a Ounce (oz)?
Formal Definition
The ounce (symbol: oz) is a unit of mass in the imperial and US customary systems. The avoirdupois ounce, the most commonly used ounce, is defined as exactly 28.349523125 grams or 1/16 of an avoirdupois pound. There are 16 ounces in one pound. The ounce is used primarily in the United States and United Kingdom for everyday mass measurements of small to moderate quantities.
Types of Ounces
Several different ounces exist. The avoirdupois ounce (28.35 g) is the standard for general commerce. The troy ounce (31.1035 g) is used for precious metals. The fluid ounce is a unit of volume, not mass (1 US fluid ounce ≈ 29.57 mL; 1 Imperial fluid ounce ≈ 28.41 mL). When "ounce" is used without qualification, it almost always means the avoirdupois ounce.
Practical Scale
An ounce is roughly the mass of a slice of bread, a standard letter in an envelope, or a AA battery. One ounce of cheese is approximately the size of a pair of dice. Six ounces is a typical serving of yogurt. The ounce provides a convenient unit for measuring food portions, postal weights, and consumer products in countries that use the imperial system.
Etymology
Latin Origins
The word "ounce" comes from the Latin "uncia," meaning one-twelfth — the same root that gives us "inch" (one-twelfth of a foot). In the Roman system, the uncia was 1/12 of the Roman pound (libra). The modern avoirdupois ounce is 1/16 of a pound, having shifted from the original 1/12 relationship over centuries of evolution.
The Symbol "oz"
The abbreviation "oz" comes from the Italian "onza" (ounce), which itself derives from the Latin "uncia." The use of "oz" became standard in English commerce by the 15th century. The period is not used ("oz" not "oz.") in modern metric-aware usage, following the convention for unit symbols.
Troy vs. Avoirdupois
The troy ounce takes its name from Troyes, a city in the Champagne region of France that was a major medieval trading center. The avoirdupois ounce comes from the Anglo-Norman French "aveir de peis" (goods of weight), referring to goods sold by weight rather than by count. These two systems coexisted for centuries before the avoirdupois system became dominant for general commerce.
Precise Definition
Avoirdupois Ounce
The international avoirdupois ounce is defined as exactly 28.349523125 grams, derived from the international avoirdupois pound (exactly 0.45359237 kg) divided by 16. This definition was established by the 1959 international yard and pound agreement.
Troy Ounce
The troy ounce is defined as exactly 31.1034768 grams. It is used exclusively for precious metals (gold, silver, platinum, palladium) and gemstones. There are 12 troy ounces in a troy pound (373.24 g), making the troy pound lighter than the avoirdupois pound (453.59 g) despite the troy ounce being heavier than the avoirdupois ounce.
Fluid Ounce
The fluid ounce is a volume measure, not a mass measure. The US fluid ounce is 29.5735 mL, while the Imperial fluid ounce is 28.4131 mL. Despite the similar name, fluid ounces measure volume and regular ounces measure mass. One fluid ounce of water weighs approximately one avoirdupois ounce, but this approximate equivalence does not hold for other liquids.
History
Roman Origins
The ounce descends from the Roman uncia, which was 1/12 of the Roman libra (pound). The libra weighed about 328.9 grams, making the uncia about 27.4 grams — remarkably close to the modern avoirdupois ounce. The Roman system spread throughout Europe with the Empire and influenced virtually every subsequent Western measurement system.
Medieval Evolution
During the Middle Ages, various ounce standards existed across Europe. The Tower pound (used at the Royal Mint in London) had an ounce of about 29.2 grams. The troy ounce, originating from the fairs at Troyes, France, weighed about 31.1 grams. The avoirdupois ounce, which became standard for general trade in England by the 14th century, weighed about 28.35 grams.
Standardization
The Weights and Measures Act of 1824 established the Imperial System and defined the avoirdupois pound (and its 16-ounce subdivision) as the standard for British commerce. The troy ounce was retained for precious metals. The 1959 international agreement defined both the avoirdupois ounce and the troy ounce in exact metric terms.
Modern Status
The ounce remains in daily use in the United States for food packaging, cooking, postal services, and consumer products. The UK has largely metricated but still uses ounces informally in some contexts. Internationally, the troy ounce remains the standard for precious metal markets worldwide — gold is priced per troy ounce on the London Bullion Market, COMEX, and all other major exchanges.
Decline in the UK
The UK has been transitioning away from ounces since joining the European Economic Community in 1973. Since 2000, most goods in the UK must be sold in metric units, though dual labeling (grams and ounces) is permitted. Loose goods like fruits and vegetables are sold by the kilogram, though market vendors may still describe quantities in ounces colloquially.
Current Use
In the United States
The ounce is ubiquitous in American daily life. Food packaging shows net weight in ounces (a standard can of soda is 12 fl oz, a bag of chips might be 8 oz). Cooking measurements use ounces. Postal rates are based on ounces. Baby weight is announced in pounds and ounces. Infant formula is prepared in fluid ounces.
In Precious Metals
The troy ounce is the worldwide standard for precious metal trading. Gold, silver, platinum, and palladium are priced per troy ounce on international exchanges. As of 2024, gold trades at approximately $2,000 to $2,400 per troy ounce. This is one of the few contexts where the ounce is used globally, not just in the US and UK.
In Food and Beverages
American food labels list serving sizes in ounces or fluid ounces. A serving of meat is typically 3 to 4 ounces. A glass of wine is typically 5 fluid ounces. Beverage container sizes (12 oz cans, 16 oz bottles, 20 oz bottles) define the American beverage market.
In the UK
British usage of ounces has declined significantly. Most packaging shows grams, though some products include ounces as supplementary information. Older Britons may still think in ounces for cooking and food shopping, but younger generations predominantly use grams.
Everyday Use
Cooking and Baking
American recipes specify ingredients in ounces: 8 oz of cream cheese, 6 oz of chocolate chips, 4 oz of butter. Measuring cups are marked in fluid ounces. Kitchen scales display weight in ounces. Cookbooks and recipe websites aimed at American audiences assume ounce-based measurements.
Baby Care
In the US, newborn babies are weighed in pounds and ounces ("7 pounds, 6 ounces"). Infant formula and breast milk are measured in fluid ounces. Baby bottles are graduated in fluid ounces (2, 4, 6, 8 oz). Pediatric growth tracking uses pounds and ounces.
Postal Services
US Postal Service rates are based on weight in ounces for First-Class Mail. A standard letter (1 oz) costs one stamp. Each additional ounce costs extra. Understanding the ounce weight of mail helps Americans estimate postage costs.
Beverages
American beverage culture is built on ounces. A "tall" coffee at Starbucks is 12 oz, a "grande" is 16 oz, and a "venti" is 20 oz. Beer is sold in 12 oz cans and bottles, 16 oz pints, and 32 or 64 oz growlers. These fluid ounce measurements define portion sizes.
In Science & Industry
Precious Metal Analysis
The troy ounce is used in assaying — the analysis of precious metal content. Assay results report gold, silver, or platinum content in troy ounces per ton of ore. Mining companies report reserves in millions of troy ounces. This is one of the few scientific/industrial contexts where the ounce is used internationally.
Limited Scientific Use
The avoirdupois ounce has virtually no role in modern science. All scientific measurement uses SI units (grams, kilograms). The ounce appears only when converting historical data or communicating results to non-technical American audiences.
Forensic Science
In US law enforcement and forensic science, drug quantities are often described in ounces for legal purposes (an "eighth" is 1/8 ounce, about 3.5 grams). This convention exists because US drug scheduling and sentencing guidelines reference ounce-based quantities.
Nutrition Science
US nutrition research sometimes reports food intake in ounces when communicating to the public, while using grams in scientific publications. The USDA dietary guidelines recommend food quantities in ounce-equivalents (e.g., 5-6.5 ounce-equivalents of protein foods per day).
Interesting Facts
The troy ounce (31.1 g) is heavier than the avoirdupois ounce (28.35 g), but the troy pound (12 troy oz = 373.2 g) is lighter than the avoirdupois pound (16 avdp oz = 453.6 g). This paradox confuses people regularly.
Gold is priced per troy ounce worldwide. At $2,300 per troy ounce (2024), a single ounce of gold is worth about 100,000 times more than an ounce of iron.
The abbreviation "oz" comes from the Italian word "onza," not from any English word. It was adopted into English commercial practice during the medieval period when Italian merchants dominated European trade.
A US quarter (25-cent coin) weighs exactly 5.670 grams, or about 0.2 avoirdupois ounces. Five quarters weigh almost exactly one ounce (28.35 g vs. 28.35 g — the match is near-perfect).
The difference between the US and UK fluid ounce is about 4%: 29.57 mL (US) vs. 28.41 mL (UK). This means a US pint (16 fl oz = 473 mL) is smaller than a UK pint (20 fl oz = 568 mL).
The word "ounce" and the word "inch" both derive from the same Latin root: "uncia" (one-twelfth). An ounce was 1/12 of a Roman pound, and an inch was 1/12 of a Roman foot.
A standard sheet of US letter paper weighs approximately 0.16 to 0.18 ounces (4.5 to 5 grams). You can mail about 5 sheets plus an envelope for one ounce of First-Class postage.
The world's annual gold mining production is approximately 3,600 tonnes, or about 115.7 million troy ounces. At current prices, this production is worth over $250 billion.
Regional Variations
The United States
The avoirdupois ounce is deeply embedded in American life. Food packaging, cooking, postal services, and consumer products all use ounces. Fluid ounces define beverage sizes. There are no current plans to replace ounces with grams in everyday American commerce.
The United Kingdom
The UK has largely transitioned to metric units for commerce. Since 2000, most goods must be sold in metric quantities, though supplementary imperial labeling (including ounces) is permitted. Ounces remain in casual conversation, particularly among older Britons.
Precious Metals Worldwide
The troy ounce is the universal standard for precious metal markets regardless of country. The London gold fix, New York COMEX, Shanghai Gold Exchange, and Tokyo Commodity Exchange all quote prices per troy ounce. This makes the ounce one of the few imperial units with truly global significance.
Other Countries
Canada officially uses grams but ounces appear on some packaging for products also sold in the US. Australia and New Zealand completed metrication in the 1970s and rarely use ounces. Most other countries use grams exclusively for everyday mass measurement.