Pound (mass)

The pound or pound-mass is a unit of mass used in British imperial and United States customary systems of measurement. Various definitions have been used; the most common today is the international avoirdupois pound, which is legally defined as exactly , and which is divided into 16 avoirdupois ounces. The international standard symbol for the avoirdupois pound is lb; an alternative symbol is lb_m (for most pound definitions), # ( chiefly in the U.S.), and or ″̶ (specifically for the apothecaries' pound).

The unit is descended from the Roman (hence the abbreviation "lb"). The English word ''pound'' is cognate with, among others, German , Dutch , and Swedish . These units are historic and are no longer used (replaced by the metric system).

Usage of the unqualified term ''pound'' reflects the historical conflation of mass and weight. This accounts for the modern distinguishing terms ''pound-mass'' and ''pound-force''.

Etymology

The word 'pound' and its cognates ultimately derive from a borrowing into Proto-Germanic of the Latin expression ('the weight measured in '), in which the word is the ablative singular of the Latin noun ('weight').

Current use

The United States and countries of the Commonwealth of Nations agreed upon common definitions for the pound and the yard. Since 1 July 1959, the international avoirdupois pound (symbol lb) has been defined as exactly .

In the United Kingdom, the use of the international pound was implemented in the Weights and Measures Act 1963.

An avoirdupois pound is equal to 16 avoirdupois ounces and to exactly 7,000 grains. The conversion factor between the kilogram and the international pound was therefore chosen to be divisible by 7 with a terminating decimal representation, and an (international) grain is thus equal to exactly .

In the UK, the process of metrication and European units of measurement directives were expected to eliminate the use of the pound and ounce, but in 2007 the European Commission abandoned the requirement for metric-only labelling on packaged goods there, and allowed for dual metric–imperial marking to continue indefinitely. When used as a measurement of body weight, common UK practice outside medical settings remains to use the stone of 14 pounds as the primary measure e.g. "11 stone 4 pounds", rather than "158 pounds" (as done in the US), or "72 kilograms" as used elsewhere.

In the United States, the Metric Conversion Act of 1975 declared the metric system to be the “preferred system of weights and measures” but did not suspend use of United States customary units, and the United States is the only industrialised country where commercial activities do not predominantly use the metric system, despite many efforts to do so, and the pound remains widely used as one of the key customary units.

Historical use

Historically, in different parts of the world, at different points in time, and for different applications, the pound (or its translation) has referred to broadly similar but not identical standards of mass or force.

Roman

The (Latin for 'scale'/'balance') is an ancient Roman unit of mass that is now equivalent to 328.9 grams. It was divided into 12 (singular: ), or ounces. The is the origin of the abbreviation for pound, "lb".

In Britain

A number of different definitions of the pound have historically been used in Britain. Among these were the avoirdupois pound and the obsolete Tower, merchant's and London pounds. Troy pounds and ounces remain in use only for the weight of certain precious metals, especially in the trade; these are normally quoted just in ounces (e.g. "500 ounces") and, when the type of ounce is not explicitly stated, the troy system is assumed.

Historically, the pound sterling was a Tower pound of silver. In 1528, the standard was changed to the Troy pound.

Avoirdupois pound

The avoirdupois pound, also known as the wool pound, first came into general use c. 1300. It was initially equal to 6992 troy grains. The pound avoirdupois was divided into 16 ounces. During the reign of Queen Elizabeth, the avoirdupois pound was redefined as 7,000 troy grains. Since then, the grain has often been an integral part of the avoirdupois system. By 1758, two Elizabethan Exchequer standard weights for the avoirdupois pound existed, and when measured in troy grains they were found to be of 7,002 grains and 6,999 grains.

=Imperial Standard Pound

=
In the United Kingdom, weights and measures have been defined by a long series of Acts of Parliament, the intention of which has been to regulate the sale of commodities. Materials traded in the marketplace are quantified according to accepted units and standards in order to avoid fraud. The standards themselves are legally defined so as to facilitate the resolution of disputes brought to the courts; only legally defined measures will be recognised by the courts. Quantifying devices used by traders (weights, weighing machines, containers of volumes, measures of length) are subject to official inspection, and penalties apply if they are fraudulent.

The Weights and Measures Act 1878 marked a major overhaul of the British system of weights and measures, and the definition of the pound given there remained in force until the 1960s. The pound was defined thus (Section 4) "The ... platinum weight ... deposited in the Standards department of the Board of Trade ... shall continue to be the imperial standard of ... weight ... and the said platinum weight shall continue to be the Imperial Standard for determining the Imperial Standard Pound for the United Kingdom". Paragraph 13 states that the weight of this standard shall be called the Imperial Standard Pound, and that all other weights mentioned in the act and permissible for commerce shall be ascertained from it alone. The First Schedule of the Act gave more details of the standard pound: it is a platinum cylinder nearly high, and diameter, and the edges are carefully rounded off. It has a groove about from the top, to allow the cylinder to be lifted using an ivory fork. It was constructed following the destruction of the Houses of Parliament by fire in 1834, and is stamped "P.S. 1844, 1 lb" (P.S. stands for "Parliamentary Standard").

= Redefinition in terms of the kilogram

=
The British 1878 Act said that contracts worded in terms of metric units would be deemed by the courts to be made according to the Imperial units defined in the Act, and a table of metric equivalents was supplied so that the Imperial equivalents could be legally calculated. This defined, in UK law, metric units in terms of Imperial ones. The equivalence for the pound was given as 1 lb = or 0.45359 kg, which made the kilogram equivalent to about .

In 1883, it was determined jointly by the standards department of the British Board of Trade and the Bureau International that was a better approximation, and this figure, rounded to was given legal status by an Order in Council in May 1898.

In 1959, based on further measurements and international coordination, the International Yard and Pound Agreement defined an "international pound" as being equivalent to exactly . This meant that the existing legal definition of the UK pound differed from the international standard pound by . To remedy this, the pound was again redefined in the United Kingdom by the Weights and Measures Act 1963 to match the international pound, stating: "the pound shall be 0.453 592 37 kilogramme exactly", a definition which remains valid to the present day.

The 2019 redefinition of the SI base units means that the pound is now defined precisely in terms of fundamental constants, ending the era of its definition in terms of physical prototypes.

Troy pound

A troy pound (abbreviated lb t) is equal to 12 troy ounces and to 5,760 grains, that is exactly grams. Troy weights were used in England by jewellers. Apothecaries also used the troy pound and ounce, but added the drachms and scruples unit in the Apothecaries' system of weights.

Troy weight may take its name from the French market town of Troyes in France where English merchants traded at least as early as the early 9th century.

The troy pound is no longer in general use or a legal unit for trade (it was abolished in the United Kingdom on 6 January 1879 by the Weights and Measures Act of 1878), but the troy ounce, of a troy pound, is still used for measurements of gems such as opals, and precious metals such as silver, platinum and particularly gold.

Tower pound

The system called Tower weight was the more general name for King Offa's pound. This dates to AD 757 and was based on the silver penny.

The Tower pound was also called the Moneyers' Pound (referring to the Saxon moneyers before the Conquest), the easterling pound, which may refer to traders of eastern Germany, or to traders on the shore of the eastern Baltic sea, or dealers of Asiatic goods who settled at the Steelyard wharf; and the Rochelle Pound by French writers, because it was also in use at Rochelle. An almost identical weight was employed by the Germans for weighing gold and silver.

The mercantile pound (1304) of 6750 troy grains, or 9600 Tower grains, derives from th