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The avoirdupois (IPA: /ˌævərdəˈpɔɪz/; French IPA: [avwɑrdypwɑ]) system is a system of weights (or, properly, mass) based on a pound of sixteen ounces. It is the everyday system of weight used in the United States. It is still widely used by many people in Canada and the United Kingdom despite the official adoption of the metric system, including the compulsory introduction of metric units in shops. It is considered^{[citation needed]} more modern than the alternative troy or apothecary or the medieval English mercantile and Tower systems.

The word avoirdupois is from French and Middle English (Anglo-French) avoir de pois, "goods of weight" or "goods sold by weight", and from Old French aveir de peis, literally "goods of weight", from aveir, "property, goods" (from aveir, "to have", from Latin habere, "to have, to hold, to possess property") + de, "from" (from the Latin) + peis, "weight", from Latin pensum, "weight". This term originally referred to a class of merchandise: aveir de peis, "goods of weight", things that were sold in bulk and were weighed on large steelyards or balances. Only later did it become identified with a particular system of units used to weigh such merchandise. The imaginative orthography of the day and the passage of the term through a series of languages (Latin, Anglo-French and English) has left many variants of the term, such as haberty-poie and haber de peyse. (The Norman "peis" became the Parisian "pois". In the 1600s "de" was replaced with "du".)

These are the units in their original French forms:

When people in Britain began to use this system they included the stone, which was eventually defined as fourteen avoirdupois pounds. The quarter, hundredweight, and ton were altered, respectively, to 28 lb, 112 lb, and 2240 lb in order for masses to be easily converted between them and stone. The following are the units in the British or imperial adaptation of the avoirdupois system:

The Thirteen British colonies in North America (not including those that formed Canada), however, adopted the French system as it was. In the U.S., quarters, hundredweights, and tons remain defined as 25, 100, and 2000 lb (though the quarter is virtually unused, as is the hundredweight outside of agriculture and commodities); if disambiguation is required then tons are referred to as the "short" units, as opposed to the British "long" units.

In the avoirdupois system, all units are multiples or fractions of the pound, which is now defined as 0.45359237 kg in most of the English-speaking world since 1959. (See the Mendenhall Order for references)

Due to the ambiguous meanings of "weight" as referring to both mass and force, it is sometimes erroneously asserted that the pound is only a unit of force. However, as defined above the pound is a unit of mass, which agrees with common usage. Also see pound-force and pound-mass.

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• French units of measurement
• Apothecaries' system of mass
• Imperial unit
• Troy weight
• U.S. customary units