The avoirdupois system is a system of weights defining terms such as pound and ounce. It is the everyday system of weight used in the United States and was used in the United Kingdom and elsewhere until metrication. It is considered more modern than the alternative troy or apothecary or the medieval English mercantile and Tower systems. The name derives from from an Old French term meaning literally "goods of weight", referring to goods sold by weight (as opposed to by the piece, for example).

In the avoirdupois system, all units are multiples or fractions of the pound, which is defined as 0.45359237 kg in most of the English-speaking world since 1959.

These are the units in their original French forms:

• 16 drams/drachms = 1 ounce (oz.)
• 16 ounces = 1 pound (lb.)
• 25 pounds = 1 quarter (qtr.)
• 4 quarter = 1 hundredweight (cwt.)
• 20 hundredweight = 1 ton/tonne

Britain, when it began to use this system, added 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 British colonies in North America, however, adopted the system as it was. In the U.S., qtrs., cwts., and tons remain defined as 25, 100, and 2000 lb (though the two former are virtually unused); they are referred to as the "short" units, as opposed to the British "long" units.

The following are the units in the British adaptation of the avoirdupois system:

• 16 drams/drachms = 1 ounce (oz.)
• 16 ounces = 1 pound (lb.)
• 14 pounds = 1 stone (st.)
• 2 stone = 1 quarter (qtr.)
• 4 quarter = 1 hundredweight (cwt.)
• 20 hundredweight = 1 ton/tonne

See also

• Apothecaries' system of mass
• Imperial unit
• Troy weight
• US customary units