Once en Gramme Convertisseur

An ounce (symbol: oz) is a unit of mass in the imperial and US customary systems of measurement. The avoirdupois ounce, which is the most commonly used form, is defined as exactly 28.349523125 grams. There are 16 ounces in one pound, making the ounce one of the fundamental units of the imperial weight system used daily in the United States, United Kingdom, and other countries that retain imperial measurements. The term "ounce" can refer to several different units depending on context. The avoirdupois ounce (28.35 g) is used for most everyday weight measurements including food, body weight, and commerce. The troy ounce (31.1035 g) is used exclusively for precious metals such as gold, silver, and platinum. The fluid ounce is a separate unit of volume, not mass, though it shares the name. For cooking and food labeling, the avoirdupois ounce is the standard in the United States. Nutritional information on US food packaging is commonly given per ounce or per 28g serving, reflecting the near-equivalence of one ounce to 28 grams.

A gram (symbol: g) is a metric unit of mass equal to one one-thousandth of a kilogram (0.001 kg). It is part of the International System of Units (SI) and is widely used worldwide for measuring small to moderate quantities of mass. One gram is approximately equal to 0.035274 avoirdupois ounces, making the conversion between grams and ounces a common requirement for international cooking and commerce. The gram provides a convenient scale for everyday measurements that are too small for kilograms but too large for milligrams. Foods, medications, precious stones, and small components are commonly measured in grams. In nutritional science, macronutrients such as protein, carbohydrates, and fat in food are expressed in grams per serving. In scientific contexts, the gram serves as the base unit of mass in the older CGS (centimeter-gram-second) system, which is still used in some specialized fields. The gram's decimal relationship to the kilogram makes it easy to use within the metric system.

The ounce has ancient roots, tracing back to the Roman uncia, which was one-twelfth of the Roman pound (libra). The word "ounce" derives directly from uncia, as does the word "inch" — both representing one-twelfth of their respective base units in the Roman system. This dual origin explains why the abbreviation for ounce is "oz," coming from the Italian "onza." During the medieval period, various ounce standards coexisted in Europe. The troy system, originating in the French city of Troyes (a major medieval trading center), used 12 ounces per pound and became the standard for precious metals and medicines. The avoirdupois system, developed for general commerce, established 16 ounces per pound and became dominant for everyday goods. In 1959, the international yard and pound agreement standardized the avoirdupois ounce at exactly 28.349523125 grams across the United States, United Kingdom, Canada, Australia, New Zealand, and South Africa. This international standard remains in use today, ensuring consistency in trade and manufacturing across countries that use the imperial system.

The gram was introduced in 1795 as part of the French metric system, the predecessor to the modern SI system. It was originally defined as the mass of one cubic centimeter of water at the temperature of melting ice (0°C). This elegant definition tied mass directly to a measurable physical quantity, making the gram both practical and scientifically grounded. When the metric system was formally established, the kilogram became the base unit of mass, with the gram defined as one one-thousandth of a kilogram. A physical prototype — the International Prototype Kilogram (IPK) — was created in 1889 to embody the standard, and the gram was defined relative to it. In 2019, the kilogram was redefined in terms of the Planck constant, a fundamental constant of quantum mechanics, making the definition of the gram equally precise and reproducible anywhere in the world without reliance on a physical artifact. This redefinition ensures the long-term stability and international consistency of the gram.

The ounce remains the primary unit of small mass measurement in the United States for everyday applications including food portions, cooking ingredients, and packaged goods. US recipes, nutritional labels, and food packaging routinely express quantities in ounces. Body weight is typically expressed in pounds and ounces for newborns and small weight differences. In international trade, ounces appear frequently on product labels sold in the US market, often alongside metric equivalents. The troy ounce is universally used in precious metals markets worldwide, regardless of whether a country uses metric or imperial measurements. Jewelers and bullion dealers globally quote gold and silver prices per troy ounce.

The gram is the standard unit for measuring food ingredients in most countries that use the metric system. Recipes in Europe, Australia, and much of Asia specify ingredients in grams, providing precise measurements that don't depend on the density of the ingredient (unlike volume measurements). Nutritional information on food packaging worldwide lists serving sizes and nutrient content in grams. In medicine and pharmacy, drug dosages are specified in grams or milligrams. Scientific research across chemistry, biology, and physics relies on grams for quantifying small masses. The gram is also the standard unit for measuring gemstones in the carat system (1 carat = 0.2 grams) and for measuring chemicals, spices, and precious materials.