El sistema de pesos avoirdupois (/ˌævərdəpɔɪz/, pronunciación francesa: [avwaʁdypwɑ]) es un sistema de pesos (o, más propiamente, de masas) establecido con base en la libra de 16 onzas. Es el sistema de pesos usado habitualmente en los Estados Unidos y sigue siendo ampliamente utilizado en diversos grados por muchas personas en Canadá, el Reino Unido, y algunas otras antiguas colonias británicas, a pesar de la adopción oficial del Sistema métrico. Un sistema alternativo de masas se ​​utiliza generalmente para materiales preciosos en joyería y orfebrería, el peso troy.

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The avoirdupois system (English pronunciation: /ˌævərdəˈpɔɪz/; abbreviated avdp) is both an updated modern as well as an historic measurement system of weights (using units such as pounds and ounces) that became commonly used in the 13th century, but was updated (1959) to represent, (by law) mass in line with SI based units. The avoirdupois weight system's general attributes were originally developed for the international wool trade in the Late Middle Ages era when trade was in recovery, and historically based on a physical standardized pound or 'prototype weight' that could be partitioned into 16 ounces. There were a number of such competing measures, and the fact the avoirdupois pound had three even numbers as multiples (half and half and half again) caused much of its popularity so that the system won out over systems with 12 or ten or 15 subdivisions. The use of this unofficial system gradually stabilized and evolved with only slight changes in the reference standard or prototype's actual mass. Over time, the desire to use only a few such scales allowed establishment of value relationships to other commodities metered and sold by weight measurements such as bulk goods (grains, ores, flax) and smelted metals, so gradually became an accepted standard through much of Europe. In England, Henry VII authorized its use as a standard and Queen Elizabeth I acted three times to ensure a common standard establishing what became the Imperial system of weights and measures. Late in the 19th century various governments acted to redefine their base standards on a scientific basis and establish ratio-metric equations to SI metric system standards. They did not always pick the same equivalencies, though the pound remained very similar; these independent legal actions lead to slight (small fractional) differences in certain quantities, such as the American and Imperial pounds, et al. In 1959 by international agreement, the definition of such pound and ounce units became standardized among countries using the pound-based mass units. The International Avoirdupois Pound so created is the everyday system of weight used in the United States and is still used to varying degrees in everyday life in the United Kingdom, Canada and some other former British colonies despite the official adoption of the metric system. An alternative system of mass, the troy system, is generally used for precious materials. The modern definition of the avoirdupois pound is exactly 0.45359237 kilograms.

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