Avoirdupois (/ˌævərdəˈpɔɪz, ˌævwɑːrdjuːˈpwɑː/;^[1] abbreviated avdp.)^[2] is a measurement system of weights that uses pounds and ounces as units.^[3][4] It was first commonly used in the 13th century AD and was updated in 1959.^[4]

In 1959, by international agreement, the definitions of the pound and ounce became standardized in countries which use the pound as a unit of mass.^[3] The International Avoirdupois Pound was then created. It is the everyday system of weights used in the United States. It is still used, in varying degrees, in everyday life in the United Kingdom, Canada, New Zealand, Australia, and some other former British colonies, despite their official adoption of the metric system.

The avoirdupois weight system's general attributes were originally developed for the international wool trade in the Late Middle Ages, when trade was in recovery. It was historically based on a physical standardized pound or "prototype weight" that could be divided into 16 ounces.^{[lower-alpha 1]} There were a number of competing measures of mass, and the fact that the avoirdupois pound had three even numbers as divisors (half and half and half again) may have been a cause of much of its popularity, so that the system won out over systems with 12 or 10 or 15 subdivisions.^[3] The use of this unofficial system gradually stabilized^{[lower-alpha 2]} and evolved, with only slight changes in the reference standard or in the prototype's actual mass.

Over time, the desire not to use too many different systems of measurement allowed the establishment of "value relationships", with other commodities metered and sold by weight measurements such as bulk goods (grains, ores, flax) and smelted metals; so the avoirdupois system gradually became an accepted standard through much of Europe.^[3]

In England, Henry VII authorized its use as a standard, and Queen Elizabeth I acted three times to enforce a common standard, thus establishing what became the Imperial system of weights and measures.^[3] Late in the 19th century various governments acted to redefine their base standards on a scientific basis and establish ratios between local avoirdupois measurements and international SI metric system standards.^[3] The legal actions of these various governments were independently conceived, and so did not always pick the same ratios to metric units for each avoirdupois unit. The result of this was, after these standardisations, measurements of the same name often had marginally different recognised values in different regions (although the pound generally remained very similar). In the modern day, this is evident in the small difference between United States customary and British Imperial pounds.^[3]

An alternative system of mass, the troy system, is generally used for precious materials. The modern definition of the avoirdupois pound (1 lb) is exactly 0.45359237 kilograms.^[3][4]

The word avoirdupois is from Anglo-Norman French aveir de peis (later avoir du pois), literally "goods of weight" (Old French aveir, as verb meaning "to have" and as noun meaning "property, goods", comes from the Latin habere, "to have, to hold, to possess something"; de = "from"/"of", cf. Latin; peis = "weight", from Latin pensum).^[5][6] 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 the term become identified with a particular system of units used to weigh such merchandise. Inconsistent orthography throughout history has left many variants of the term, such as haberty-poie and haber de peyse. (The Norman peis became the Parisian pois. In the 17th century de was replaced with du.)^[7]

The current spelling of the last word is poids in the current standard French orthography,^[8] but the spelling avoirdupois remained as is in the anglosphere.

The rise in use of the measurement system corresponds to the regrowth of trade during the High Middle Ages after the early crusades, when Europe experienced a growth in towns, turned from the chaos of warlordism to long-distance trade, and began annual fairs, tournaments and commerce, by land and sea. There are two major hypotheses regarding the origins of the avoirdupois system. The older hypothesis is that it originated in France.^[9] A newer hypothesis is that it is based on the weight system of Florence.^[3][10]

The avoirdupois weight system is thought to have come into use in England around 1300.^{[citation needed]} It was originally used for weighing wool. In the early 14th century several other specialized weight systems were used, including the weight system of the Hanseatic League with a 16-ounce pound of 7200 grains and an 8-ounce mark.^{[citation needed]} However, the main weight system, used for coinage and for everyday use, was based on the 12-ounce tower pound of 5400 grains.^{[citation needed]} From the 14th century until the late 16th century, the system's basis and the prototype for today's international pound, the avoirdupois pound, was also known as the wool pound or the avoirdupois wool pound.

The earliest known version of the avoirdupois weight system had the following units: a pound of 6992 grains, a stone of 14 pounds, a woolsack of 26 stone, an ounce of 1⁄16 pound, and finally, the ounce was divided into 16 "parts".^[11]

The earliest known occurrence of the word "avoirdupois" (or some variant thereof) in England is from a document entitled Tractatus de Ponderibus et Mensuris ("Treatise on Weights and Measures"). This document is listed in early statute books under the heading 31 Edward I dated 2 February 1303. More recent statute books list it among statutes of uncertain date. Scholars nowadays believe that it was probably written between 1266 and 1303.^[12] Initially a royal memorandum, it eventually took on the force of law and was recognized as a statute by King Henry VIII and Queen Elizabeth I.

It was repealed by the Weights and Measures Act 1824. In the Tractatus, the word "avoirdupois" refers not to a weight system, but to a class of goods, specifically heavy goods sold by weight, as opposed to goods sold by volume, count, or some other method. Since it is written in Anglo-Norman French, this document is not the first occurrence of the word in the English language.^[13][14]

• Apothecaries' system
• Units of measurement in France
• Imperial units
• Troy weight
• United States customary units
• Weighing scales